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VACUUM  TUBES 

IN 

WIRELESS  COMMUNICATION 


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VACUUM  TUBES 

IN 

WIRELESS   COMMUNICATION 

A   PRACTICAL  TEXTBOOK    FOR 
OPERATORS  AND  EXPERIMENTERS 


BY 
ELMER    E.    BUCHER 

V\ 

Instructing  Engineer,  Marconi  Wireless  Telegraph   Company  of  America 

Member  Institute  of  Radio  Engineers.      Director  of  Instruction,  Marconi  Institute 

Author  of    Practical  Wireless  Telegraphy" 


This  volume  shows  over  100  different  circuits  for  the  practical  use  of  Vacuum 
Tubes  as  Detectors,  Radio  or  Audio  Frequency  Amplifiers,  Regenerative 
Receivers,  Beat  Receivers,  and  Generators  of  Radio  Frequency  Currents. 

A  series  of  graphic  charts  in  the  appendix  reveals  the  functioning  of  the 
vacuum  tube  in  an  elementary  manner.  The  technical  introduction  reviews 
the  problems  of  continuous  and  discontinuous  wave  transmitters  and  receivers. 


FULLY  ILLUSTRATED 


Engineering 
Library 


COPYRIGHT,  1918 

BY 
WIRELESS  PRESS,  INC. 


AUTHOR'S  NOTE 

In  preparing  the  text  of  this  book,  the  author  had  two  principal  ob- 
jects in  view:  (1)  to  provide  the  Government  and  commercial  wireless 
operator  with  a  brief  and  simple  explanation  of  the  functioning  of  the 
circuits  of  the  vacuum  tube,  (2)  to  lay  before  the  experimenter  and  the 
practical  operator  the  numerous  circuits  employed  from  time  to  time  in 
the  laboratory  and  in  commercial  practice. 

Outside  of  its  obvious  commercial  value,  the  perfected  vacuum  tube 
affords  the  experimenter  a  most  fascinating  field  of  research.  This  is 
well  evidenced  by  the  fact  that  a  single  bulb  with  associated  tuning 
apparatus  connected  to  a  four  wire  aerial  200  feet  in  length  permits 
wireless  signals  to  be  received  over  distances  of  2,500  to  4,000  miles  in 
daylight,  and  up  to  6,000  miles  in  darkness.  Any  experimenter  may 
prove  this  statement  by  adopting  one  of  the  circuits  disclosed  in  this 
volume. 

TO    THE   STUDENT   AND   EXPERIMENTER 

Many  of  the  circuits  shown  in  this  book  are  not  employed  in  either 
Government  or  commercial  wireless  communication.  Some  will  be  found 
to  be  duplicates  in  so  far  as  their  mode  of  operation  is  concerned,  the 
various  parts  of  the  receiving  apparatus  being  re-arranged  or  re-grouped. 
The  diagrams  represent,  however,  the  connections  disclosed  from  time 
to  time  and  in  order  that  the  student  may  review  step  by  step  the  pro- 
gress in  the  application  of  the  vacuum  tubes,  it  was  thought  best  to  place 
before  him  the  use  of  the  tube  in  all  its  phases. 

The  connections  most  suitable  for  different  conditions  of  wireless 
service  suggest  themselves,  once  the  fundamental  operating  principle  of 
the  vacuum  tube  is  understood. 

It  will  very  probably  develop  that  some  of  the  explanations  of  work- 
ing given  herein  both  for  the  fundamental  arrangements  and  for  sys- 
tems which  are  extensions  of  the  simple  or  fundamental  circuits  sho~vn 
in  the  first  three  chapters,  will  not  stand  the  test  of  further  research, 
but  in  general  it  may  be  said  that  the  use  of  the  vacuum  valve  as  a 
detector,  amplifier,  and  generator  of  radio  frequency  currents  is  well 
understood. 

The  systems  described  in  Chapters  III  to  VII  inclusive,  are  exten- 
sions of  the  fundamental  circuits  shown  in  Chapters  I,  II,  and  III. 
Hence,  to  understand  fully  the  mode  of  operation  of  the  more  compli- 
cated apparatus,  the  experimenter  should  read  the  preliminary  chapters. 
Any  seeming  omissions  in  the  advanced  chapters  will  probably  be  found 
covered  in  the  preliminary  text. 

New  York.  E.E.B. 

v 

382934 


CONTENTS 

INTRODUCTION 

1.  Audio  and  Radio  Frequency  Currents.  2.  Damped  and  Undamped  Oscilla- 
tions: (a)  The  Use  of  Rectifiers,  (b)  The  Detection  of  Continuous  Oscilla- 
tions, (c)  Distinction  Between  Continuous  and  Discontinuous  Wave  Systems. 

3.  Receiver  Circuits:     (a)  The  Necessity  for  Tuning,    (b)  Tuning  in  Practice. 

4.  The  Telephone  Receiver. 

PART  I 

THE  OSCILLATION  VALVE  IN  RADIOTELEGRAPHY 

5.  Foreword.     6.  What  Is  an  Electron?     7.  How  May  Electrons  Be  Set  in 
Motion?     8.  Fleming's  Discovery.     9.  The  Oscillation  Valve.     10.  Demonstra- 
tion of  Rectifying  Properties.     11.  Non-Uniform  Conductivity  of  the  Valve. 

12.  How  the  Valve  Becomes  Saturated. 

PART  II 

PRACTICAL  APPLICATIONS  OF  THE  OSCILLATION  VALVE 

13.  In  General.    14.  Action  of  the  Simple  Valve.    15.  The  Practical  Oscillation 
Valve  Circuit  in   Radio:     (a)    Fleming  Valve   Circuits  in  Simple  Form,     (b) 
Other  Circuits  for  the  Vacuum  Valve.     16.  The  Three-electrode  Valve,     (a) 
Characteristic    Curve    of    Three-Electrode    Valve.      (b)    Valve    Terminology. 
17.  The  Three-electrode  Valve  as  an  Electron  Relay  and  Rectifier:     (a)  The 
Valve  as  an  Electron  Relay  (without  Grid  Condenser),     (b)   Functioning  of 
the  Valve  as  Explained  from  the  Characteristic  Curve,     (c)  Relaying  Action, 
(d)   Use  of  the  Three-Electrode  Valve  with  Grid  Condenser,     (e)  Radio  and 
Audio    Frequency    Component    of    the    Plate    Current,      (f)    Summary    of    the 
Phenomena  of  the  Characteristic  Curve.     18.  The  Tuned  Plate  Circuit. 

PART  III 

CASCADE  AMPLIFICATION  BY  THE  VACUUM  VALVE 

19.  In  General.  20.  Cascade  Radio  Frequency  Amplifier:  (a)  Curves  of  the 
Cascade  Amplifier.  21.  Cascade  Audio  Frequency  Amplifier.  22.  Combined 
Radio  and  Audio  Frequency  Cascade  Amplifier. 

PART  IV 

THE  REGENERATIVE  VACUUM  VALVE  AMPLIFIER 

23.  In  General.  24.  Preliminary  Considerations.  25.  The  Principle  of  Regen- 
eration. 26.  Radio  Frequency  Regenerative  Circuit:  (a)  Phenomena  of  Re- 
generation, (b)  Adjustment  of  Regenerative  Amplifier.  27.  Tuned  Plate 
Circuits.  28.  Audio  Frequency  Regenerative  System.  29.  Combined  Audio 
and  Radio  Frequency  Regenerative  System.  30.  Electrostatic  and  Direct 
Magnetic  Coupling.  31.  The  "Reaction"  Circuits  of  Franklin  and  Marconi. 
32.  Simple  Regenerative  Circuit. 

vii 


PARTY 

COMBINED  REGENERATIVE  AND  CASCADE  AMPLIFICA- 
TION SYSTEMS 

33.  In  General.  34.  Regenerative  Cascade  Systems.  35.  Audio  Frequency  Re- 
generative and  Cascade  Systems. 

PART  VI 

THE  VACUUM  TUBE  AS  A  DETECTOR  OF  CONTINUOUS 

WAVES.    AUDIO  FREQUENCY  TUNING  CIRCUITS. 

SPECIAL  CIRCUITS  FOR  THE  VACUUM  TUBE. 

36.  Reception  of  Continuous  Oscillations.  37.  The  Theory  of  the  Beat  Receiver: 
(a)  Phenomenon  of  Beats.  38.  The  Simple  Heterodyne:  (a)  Curves  of  the  Beat 
Receiver  in  Radio.  39.  Selectivity  by  the  Heterodyne.  40.  Heterodyne  with 
the  Vacuum  Valve  as  a  Detector.  41.  The  Vacuum  Valve  as  a  Generator  of 
Radio  Frequency  Oscillations.  42.  The  Regenerative  Beat  Receiver:  (a)  How 
the  Valve  Is  Set  Into  Self-Oscillation:  (b)  The  Phenomena  of  the  Self- 
Heterodyne.  43.  Audio  Frequency  Tuning.  44.  The  Weagant  Undamped  Wave 
Receiver.  45.  Modified  Weagant  Beat  Receiver.  46.  Reception  of  Continuous 
Waves  by  the  Valve  Without  Beats.  47.  Cascade  Amplification.  48.  Cascade 
Regenerative  Systems  for  Beat  Reception.  49.  Beat  Reception  from  Damped 
Wave  Transmitters.  50.  Special  Circuits  for  the  Vacuum  Valve:  (a)  The 
Use  of  a  Rotary  Condenser  for  the  Reception  of  Continuous  Oscillations. 
51.  "Open  Circuit"  Oscillators  for  the  Vacuum  Valve:  (a)  Modified  Open  Cir- 
cuit Oscillators.  52.  Amplification  by  the  Heterodyne  System.  53.  Kenotron 
and  Pliotron  Vacuum  Tubes:  (a)  The  Connections  of  the  Pliotron  for  the 
Generation  of  Radio  Frequencies  at  High  Current  Values,  (b)  The  Pliotron 
as  a  Generator  of  High  Voltages  at  Radio  Frequencies. 

PART  VII 

WIRELESS  TELEPHONY 

54.  In  General:     (a)   Fundamental  Circuits  of  the  Three-Electrode  Tube  as  a 
Radiophone      Transmitter.         (b)    Rounds'      Wireless      Telephone       System. 

55.  Hund's  Method  of  Radio  Frequency  Modulation.     56.  Englund's  Wireless 
Telephone    System.       57.  Carson's    Proposed    Wireless    Telephone    System. 
58.  Espenschied's  Duplex  Wireless  Telephone  System.    59.  Englund's  Duplex 
Radio  Telephone  and  Radio  Telegraph  System. 

PART  VIII 

THE  DYNATRON  DETECTOR  AND  OSCILLATOR 

60.  The  Dynatron.  61.  The  Dynatron  as  an  Oscillator.  62.  The  Dynatron  as 
a  Detector  of  Electrical  Oscillations.  63.  The  Dynatron  as  a  Compensator  for 
Circuit  Losses.  64.  The  Pliodynatron.  65.  The  Pliodynatron.  as  a  Detector. 
66.  The  Pliodynatron  in  Radio  Telephony. 

APPENDIX 

Diagrams  Illustrating  Certain  Fundamental  Actions  of  the  Vacuum  Tube  as  an 
Oscillation  Detector  in  Radio  Telegraphy. 

viii  -.* 


VACUUM  TUBES 

IN 
WIRELESS  COMMUNICATION 

INTRODUCTION 

Before  entering  into  an  explanation  of  the  functioning  of  the  vacuum 
tube,  a  brief  review  of  certain  facts  bearing  on  wireless  transmitting  and 
receiving  systems  will  be  given. 

1.  AUDIO   AND  RADIO   FREQUENCY   CURRENTS.— It  is 

well  known  that  vibrations  above  20,000  per  second  are  practically  in- 
audible to  the  human  ear.  This  fact  has  an  important  bearing  on  the 
reception  of  radio  telegraphic  signals  by  the  usual  telephone  receiver. 
For  example,  if  a  20,000  cycle  current  flows  through  the  telephone  wind- 
ings, the  diaphragm  will  vibrate  at  the  upper  limits  of  audibility.  If  the 
frequency  of  the  applied  current  is  progressively  decreased,  the  vibrations 
of  the  diaphragm  become  more  and  more  audible,  until  some  value  is 
reached  where  the  maximum  deflection  of  the  diaphragm  is  secured  for 
a  given  E.M.F. 

Considering  the  foregoing  phenomena,  a  distinction  has  been  drawn 
between  currents  within  the  range  of  frequencies  employed  to  radiate 
electric  waves  in  wireless  telegraphy  and  those  of  lower  frequencies, 
which  it  is  believed  are  not  capable  of  setting  into  motion  electric  waves 
(this  has  not  been  proven),  but  which  are  audible  in  the  telephone  re- 
ceiver. A  figure  of  10,000  cycles  has  been  arbitrarily  adopted  as  a  divi- 
ding line.  The  following  definition  is  therefore  of  immediate  importance : 

(1)  Currents    of    frequency    above    10,000    cycles    per    second    are 

termed  currents  of  radio  frequency. 

(2)  Currents    of    frequency    below     10,000    cycles    per    second    are 

termed  currents  of  audio  frequency. 

Now,  to  explain  more  clearly  what  the  foregoing  remarks  have  to 
do  with  the  problems  of  radio  telegraphy  we  must  remember  that  the 


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•*'  °  »-  £    "A    £    V  3  ^      ^    3  •    -5          ,'       ,,',       "       ' 

2  Vacuum  Tubes  In  Wireless  Communication 

electric  waves  for  the  dispatch  of  wireless  messages  from  ship  to  ship 
and  from  ship  to  shore  are  generated  by  extremely  high  frequency  cur- 
rents of  the  order  of  from  500,000  to  1,000,000  cycles  per  second.  For 
long  distance  communication  much  lower  frequencies  of  the  order  of 
100,000  cycles  down  to  20,000  cycles  per  second  are  employed.  Currents 
of  similar  frequency  are  induced  in  the  receiver  circuits.  But  in  all  cases 
the  frequency  of  the  current  is  above  audition;  hence  we  see  that  the 
telephone  receiver  cannot  be  employed  as  an  indicator  of  the  passage  of 
such  currents. 

Keeping  in  mind  that  an  audio  frequency  current  of  from  200  to 
1000  impulses  per  second  gives  audible  response  with  a  minimum  of 
current*,  and  because  the  telephone  is  almost  invariably  employed  as  a 
current  translator  in  radio,  we  see  that  the  problem  of  detection  in  wire- 
less reception,  simply  resolves  itself  into  one  of  converting  radio  fre- 
quency currents  into  audio  frequency  currents  such  as  will  give  the 
best  response  in  a  telephone."^ 

These  audio  frequency  currents  are  not  necessarily  simple  alternating 
currents.  They  may  consist  of  groups  of  direct  current  pulses  or  they 
may  be  complex  audio  frequency  currents  of  various  wave  forms ;  what- 
ever their  nature,  so  long  as  the  telephone  diaphragm  is  impulsed  from 
say  200  to  1,000  times  per  second,  the  best  response  will  be  secured  from 
a  given  E.M.F. 

Devices  which  effect  this  conversion  are  termed  oscillation  detectors, 
— of  which  there  are  many  types.  The  functioning  of  certain  detectors 
will  be  briefly  reviewed  further  on. 

2.    DAMPED    AND    UNDAMPED    OSCILLATIONS.— There 

are  two  general  systems  of  radio  transmission:  the  discontinuous  wave 
system,  and  the  continuous  wave  system.  The  currents  by  which  these 
waves  are  set  into  motion  are  known  as  damped  and  undamped  oscilla- 
tions respectively.  In  the  discontinuous  system,  the  radio  frequency 
currents  are  generated  in  audio  frequency  groups  of  120  to  1,000  per 
second.  Such  currents  are  obtained  by  the  charge  and  discharge  of  a 
condenser  through  a  circuit  containing  an  inductance. 

Continuous  oscillations  are  generated  by 

(1)  the  radio  frequency  alternator; 

(2)  the  direct  current  arc  generator; 

(3)  the  vacuum  tube  oscillator; 

(4)  by  overlapping  trains  of  damped  oscillations. 


*TMs  statement  applies  to  a  sinusoidal  E.M.F. 

tThis,  of  course,  is  not  true  when  a  measuring  instrument  is  employed  to 
detect  the  incoming  oscillations  instead  of  a  telephone. 


Vacuum  Tubes  In  Wireless  Communication  3 

Several  groups  of  discontinuous  oscillations  are  shown  in  Figure  1. 
Continuous  oscillations  are  indicated  in  figure  2.  Certain  fundamental 
distinctions  in  the  two  systems  of  radio  transmission,  particularly  in 
regard  to  the  receiving  apparatus,  are  to  be  noted. 

Let  it  be  understood  that  the  successive  groups  of  radio  frequency 
oscillations  shown  in  figure  1  are  generated  by  condenser  discharges 
occurring  at  audio  frequency.  Currents  of  similar  frequency  flow  in  the 
transmitter  aerial,  part  of  the  energy  being  detached  in  the  form  of  elec- 


1.000,000  SEC> 

Figure  1 

Discontinuous   or    damped   oscillations   such    as   generated   by   the 
periodic  charge  and  discharge  of  a  condenser  across  a  spark  gap. 

tromagnetic  waves.  These  waves  generate  in  the  receiver  aerial  cur- 
rents of  the  frequency  of  the  transmitter  which  in  turn  are  impressed 
upon  a  local  detector  circuit.  The  frequency  of  the  individual  cycles 
1,  2,  3,  4,  per  group  are  above  audibility  and  cannot  be  detected 
directly  in  a  telephone,  but  by  various  means  these  groups  may  be 
modified  to  a  form  suitable  to  actuate  the  telephone  diaphragm  as  will 
be  explained  in  the  paragraph  following. 

(a)  The  Use  of  Rectifiers.  Audible  response  in  a  telephone  may 
be  obtained  from  group  frequency  (damped  wave)  transmitters  by  what 
is  known  as  a  rectifier  of  alternating  currents.  If  such  a  device  is  con- 
nected in  series  with  the  telephone  and  an  oscillating  E.M.F.  is  impressed 
upon  the  circuit,  one  half  of  the  incoming  cycle  will  pass  the  rectifier 
but  the  other  half  will  be  practically  suppressed.  Then,  for  each  group 
of  incoming  oscillations  (set  up  in  the  receiver  aerial  by  the  distant 
transmitter)  a  group  of  decaying  direct  current  pulses  will  flow  through 
the  telephone,  the  average  effect  of  three  groups  being  illustrated  by  the 
graphs  A,  B,  C,  Figure  3. 

Among  the  crystalline  elements  possessing  the  ability  to  rectify  high 
frequency  currents  may  be  mentioned  carborundum,  galena,  silicon, 
molybdenite,  iron  pyrites,  zincite-bornite. 

Rectification  also  may  be  obtained  by  using  the  one-way  conduc- 


Vacuum  Tubes  In  Wireless  Communication 


tivity  between  a  hot  lamp  filament  and  a  metallic  element  inserted  in  a 
vacuum,  i.e.,  the  vacuum  tube;  or  we  may  partially  rectify  radio  fre- 
quency currents  mechanically  as,  for  example,  by  driving  a  Goldschmidt 
tone  wheel  out  of  synchronism  with  the  incoming  oscillations.  In  detec- 
tors used  with  a  local  battery  and  having  suitable  operating  character- 
istics, the  rectification  may  be  accompanied  by  a  relaying  action  and 


SEC 


1,000,000 

Figure  2 

Oscillations  of  constant  .  amplitude   (continuous  or  undamped  oscil- 
lations). 

amplification  of  signals  may  thus  be  secured.  This  is  particularly  true 
of  vacuum  valve  detectors. 

Oscillation  detectors  suitable  for  the  reception  of  damped  oscilla- 
tions but  now  chiefly  of  historical  interest  are  the  coherer,  and  the  micro- 
phone detector,  both  indicating  by  virtue  of  a  change  in  their  resistance, 
the  passage  of  radio  frequency  currents.  The  microphone  possessed  the 
advantage  over  the  coherer,  in  that  its  sensitiveness  was  automatically  re- 
stored after  the  reception  of  a  signal;  the  coherer  required  a  tapper  to 
place  the  tube  in  operating  condition  after  the  passage  of  each  signal. 
This  made  it  particularly  sluggish  in  action. 

The  Marconi  magnetic  detector  takes  as  its  basis  of  operation  the 
change  wrought  in  the  magnetization  of  a  moving  band  of  iron  (possibly 
due  to  magnetic  hysteresis),  by  the  magnetic  field  generated  by  the  in- 
coming radio  frequency  oscillations.  This  change  in  flux  in  turn  induces 
an  audio  frequency  current  in  a  small  coil  of  wire,  in  inductive  relation 
to  the  iron  band,  to  which  is  connected  a  telephone  of  about  75  ohms 
resistance. 

In  this  method  of  reception  as  with  the  crystal  rectifiers,  the  tele- 
phone diaphragm  is  impulsed  once  for  each  spark  at  the  transmitter. 

(b)  The  Detection  of  Continuous  Oscillations.  If  we  connect  a 
rectifier  and  a  telephone  in  series  with  a  radio  frequency  circuit  in 
which  flow  the  continuous  oscillations  shown  in  Figure  2,  it  is  clear  that 
the  telephones  will  pass  a  pulsating  direct  current  of  continuous  ampli- 
tude. The  individual  pulses  occur  at  frequencies  above  audibility  and 


Vacuum  Tubes  In  Wireless  Communication  5 

no  sound  will  therefore  be  produced  except  at  the  opening  and  closing 
of  the  circuit.  It  is  necessary  then,  to  modify  further  the  resultant  cur- 
rent in  order  to  secure  audible  response,  as  will  be  seen  further  on. 

Among  the  many  methods  through  which  continuous  radio  frequency 
oscillations  may  be  converted  to  audio  frequency  currents,  we  may  take 
as  an  example  the  tikker,  which  is  simply  a  mechanical  circuit  inter- 


A 


A  A  RECTIFIED 

A  A   CURRENTS 

I1AAA      MA  A 


Figure  3 

Graphs  showing  how  incoming  radio  frequency 
currents  are  converted  to  direct  current  pulses  by  an 
oscillation  detector. 


rupter  connected  in  some  part  of  the  receiving  system  to  interrupt  the 
incoming  oscillations  200  to  1,000  times  per  second.  The  resulting 
groups  (of  radio  frequency  current)  in  turn  energize  an  audio  fre- 
quency circuit  consisting  of  a  condenser  with  a  telephone  in  shunt,  or, 
to  improve  the  tone,  a  crystal  rectifier  is  often  inserted  in  series  with  the 
tikker  to  rectify  these  groups. 

By  means  of  the  slipping  contact  detector,  continuous  oscillations 
are  made  audible  by  virtue  of  the  periodic  changes  in  resistance  of  some 
part  of  the  receiver  circuit.  Incoming  radio  frequency  currents  are  thus 
modulated  to  an  audio  frequency  current  which  is  fairly  uniform. 

A  valuable  method  for  producing  an  audio  frequency  current  from 
continuous  oscillations  is  the  system  which  generates  what  are  termed 
beat  currents.  In  brief,  if  two  radio  frequency  currents  of  slightly  dif- 
ferent frequencies  be  impressed  upon  the  same  circuit,  they  successively 
assist  and  oppose,  going  in  and  out  of  phase  progressively.  The  result 
is  the  production  of  a  third  current  called  a  beat  current,  which,  if  the 
frequencies  are  properly  selected,  may  occur  at  an  audio  frequency. 
Now,  one  of  these  radio  frequencies  may  be  that  of  the  incoming  signal 
and  the  other  may  be  generated  locally.  By  proper  selection  of  the  two 


6  Vacuum  Tubes  In  Wireless  Communication 

frequencies,  beat  currents  occurring  at  an  audible  frequency  suitable 
for  maximum  response  from  the  head  telephone  will  be  generated  in 
the  receiving  system.  These,  if  they  undergo  rectification  in  some  type 
of  oscillation  detector,  will  actuate  the  telephone  diaphragm. 

(c)   Distinction   Between   Continuous   and   Discontinuous   Wave 
Systems.    The  principal  points  of  difference  between  the  damped  and  un- 


0-3 


0-4 


Figure  4 

Graphs  showing  the  phenomena  involved  in  the  transmission  and  re- 
ception of  wireless  signals  in  spark  systems  of  radio  telegraphy. 


damped  wave  systems  and  particularly  the  problems  involved  in  the 
detection  of  currents  in  either  system,  at  the  receiving  station,  may  be 
explained  graphically  by  the  curves  of  Figures  4  and  5. 

In  Figure  4,  the  graphs  0-1  to  0-4  outline  certain  phenomena  con- 
cerning the  transmission  and  detection  of  electric  waves  in  the  discon- 


Vacuum  Tubes  In  Wireless  Communication  7 

tinuous  wave  system.  Graph  0-1*  indicates  one  cycle  of  charging  current 
such  as  is  impressed  across  the  transmitter  condenser  by  a  high  voltage 
transformer.  The  frequency  of  the  current  is  500  cycles  per  second. 
It  is  assumed  that  the  condenser  is  charged  twice  for  each  cycle.  There- 
fore a  500-cycle  generator  (and  transformer)  will  energize  the  condenser 
circuit  1,000  times  per  second  and  1,000  sparks  will  discharge  across  the 
gap.  The  discharge,  as  is  well  known,  gives  rise  to  radio  frequency 
currents. 

Each  alternation  of  the  charging  current,  therefore,  releases  in  the 
closed  oscillation  circuit  one  group  of  radio  frequency  oscillations  which, 
through  a  coupling  coil  are  induced  in  the  antenna  circuit  giving  the 
oscillations  shown  by  graph  0-2.  Currents  of  similar  frequency  are  im- 
pressed across  the  oscillation  detector  (rectifier)  at  the  receiving  station, 
but  their  frequency  is  too  high  for  direct  response  in  the  head  tele- 
phone. Now,  if  a  rectifier  is  connected  in  the  circuit  converting  these 
currents  into  a  series  of  semi-cycles  as  in  the  graph  0-3,  no  response 
will  be  obtained  in  the  telephone  from  the  individual  pulses,  because 
they  occur  at  rates  above  audition,  but  since  they  flow  in  the  same 
direction,  the  diaphragm  responds  to  an  average  effect  as  shown  by  the 
graph  0-4.  Each  spark  discharge  at  the  transmitter  therefore  event- 
ually sends  a  uni-directional  current  pulse  through  the  receiver  produc- 
ing one  click  or  sound. 

We  may  contrast  the  foregoing  with  the  problems  involved  in  the 
detection  of  continuous  oscillations.  In  the  continuous  wave  system  the 
transmitter  aerial  radiates  a  steady  stream  of  waves  shown  by  the  graph 
0-1,  Figure  5,  and  a  similar  oscillating  electromotive  force  is  impressed 
across  the  terminals  of  the  oscillation  detector  at  the  receiving  station. 
But  no  response  would  be  obtained  in  the  head  telephones,  for,  as 
already  explained,  the  frequency  of  the  uni-directional  pulses  0-3  is 
above  audition.  This  stream  of  oscillations  may,  however,  be  interrupted 


*The  student  should  compare  the  relative  time  periods  of  the  graphs  of 
figure  4.     For  the  audio  frequency  charging  current  of  0-1  the  period  of  one 
1 

cycle  is   of  a   second;   for  the  individual  cycle  of  the  current  impressed 

500 

1  1 

across  the  oscillation  detector  as  in  0-2  from  >  to of  a  second,  the 

25,000         1,000,000 

periods  vary  in  fact  as  the  inductance  and  capacity  of  the  oscillation  circuit. 
The  frequency  of  the  telephone  current  of  0-4  is  1,000  per  second,  but  the  dura- 

1 

tion  of  each  pulse  is  not  necessarily of  a  second,  for  obviously  its  duration 

1,000 

will  vary  as  the  number  of  cycles  in  each  group  of  radio  frequency  currents. 
(The  actual  number  of  cycles  per  spark  depends  upon  the  damping  of  the  cir- 
cuit. Nevertheless,  the  telephone  will  be  energized  1,000  times  per  second), 


8 


Vacuum  Tubes  In  Wireless  Communication 


at  the  receiving  station  say  1,000  times  per  second  by  some  form  of  the 
tikker  as  shown  by  graph  0-2.  Groups  of  rectified  currents  will  pass 
through  the  telephone  as  indicated  by  the  graph  0-3  and  the  resulting 
telephone  current  will  be  that  shown  by  graph  0-4. 

It  is  evident  that  in  the  first  case  (  Figure  4),  the  radiated  energy 
is  modulated  to  an  audio  frequency  by  the  audio-frequency  charging  cur- 
rent, but  in  the  second  case  (Figure  5),  this  audio  frequency  variation  is 
effected  at  the  receiver. 


o-i 


0-2 


0-3 


0-4 


A 


A 


A  A  A  A  A 


A  A  A  A  A 


INCOMING 
-CONTINOUS 
OSCILLATIONS 


RADIO 

FREQUENCY 

GROUPS 


YELEPHONt 
"—  CURRENT 


Figure  5 

Graphs  illustrating  how  continuous  oscillations  may  be  made  audi- 
ble in  a  receiving  telephone. 


3.  RECEIVER  CIRCUITS.— In  general,  a  modern  radio  receiv- 
ing set  comprises  two  fundamental  circuits* : 

(1)  the  open  circuit; 

(2)  the  closed  or  secondary  circuit. 

The  open  circuit  embraces  all  apparatus  from  the  antenna  to  the 
earth  connection  including  usually,  an  aerial  tuning  inductance,  the 
primary  winding  of  a  tuning  transformer,  and  a  series  variable  con- 
denser. 


*  Single  circuit  receiving  systems  have  had  some  use  but  the  "two  circuit" 
receiver  is  generally  preferred.  Three  tuning  circuits  are  recognized  in  con- 
nection with  the  vacuum  tubes,  known  as  the  primary,  secondary  and  tertiary 
circuits. 


Vacuum  Tubes  In  Wireless  Communication  9 

The  secondary  circuit  contains  the  secondary  coil,  the  shunt  sec- 
ondary condenser,  and  some  form  of  oscillation  detector  for  transform- 
ing radio  frequency  currents  into  a  wave  form  which  will  give  maximum 
response  in  the  telephone. 

A  representative  circuit  is  shown  in  Figure  6.  L-l  is  the  primary 
winding  of  a  receiving  transformer  which  acts  inductively  upon  the 
secondary  winding  L-2.  L-3  is  the  antenna  tuning  inductance,  and  0-3, 
the  short  wave  condenser.  The  winding  L-2  is  shunted  by  a  condenser 
0-1  of  variable  capacity,  which  consists  of  a  number  of  interleaved  plates 
one  set  of  which  is  stationary  and  the  other  movable. 

The  secondary  system  further  includes  the  rectifier  D,  the  telephone 
P,  and  the  shunt  condenser  C-2.  If  turns  be  added  at  coil  L-l  or  L-3 


C-l 


f 


* 


Figure  6 
Circuits  of  the  inductively  coupled  or  two  circuit  radio  receiver. 


(the  aerial  tuning  inductance),  the  receiving  aerial  responds  to  longer 
wave  lengths,  but  if  turns  be  subtracted  at  either  this  or  the  primary 
inductance,  Jr-1,  the  aerial  responds  to  shorter  wave  lengths.  Waves  less 
in  length  than  the  natural  wave  length  of  the  aerial  can  be  tuned  to 
resonance  by  inserting  the  condenser  0-3  in  series  with  the  aerial  system. 

When  radio  frequency  currents  flow  through  L-l,  an  alternating 
magnetic  field  of  similar  frequency  cuts  through  L-2  and  currents  of 
like  frequency  flow  in  circuit  L-2,  0-1,  which  is  tuned  to  resonance  to 
the  incoming  signal.  An  oscillating  electromotive  force  is  therefore  im- 
pressed across  the  detector  D,  producing  a  rectified  current.  The  head 
telephone  P,  then,  is  traversed  by  uni-directional  currents. 

The  open  and  closed  circuits  in  this  diagram  are  inductively  coupled. 


10 


Vacuum  Tubes  In  Wireless  Communication 


Conductive  or  direct  coupling  may  be  employed  as  in  Figure  7,  or  so- 
called  electrostatic  coupling  as  in  Figure  8.  In  Figure  7,  coil  P-l  acts  as 
an  auto  transformer,  but  in  Figure  8,  coils  L-l  and  L-2  are  said  to  be 
electrostatically  coupled  through  condensers  0-1  and  0-2.  An  open  cir- 
cuit secondary  may  be  employed  as  in  Figure  9.  This  circuit  is  of  par- 


c-» 


Figure    7 

Circuits    of   the    conductively    or    direct   mag- 
netically coupled  receiver.    A  single  coil  P-l  is  em- 
ployed to  transfer  energy  from  the  antenna  to  the 
detector  circuits. 

ticular  advantage  for  use  with  oscillation  detectors  whose  response  is 
dependent  upon  the  voltage  impressed  by  a  given  group  of  oscillations. 
As  it  is  well  known  maximum  potential  is  secured  at  the  free  end  of  L-2 
for  a  given  group  of  oscillations,  by  tuning  the  circuit  L-l,  L-2,  D,  P, 
to  the  frequency  of  the  incoming  oscillations.  Response  is  secured  in 


.fej 

*Z        : 
II" 

gJ 

d>                = 

g  V2 

—  —  ^x; 

^ 

£                "i, 

,    iH", 

,e^ 

g 

Figure  8 

The  electrostatically  coupled  receiver.  The  primary  and  secondary 
tuning  coils  L-l  and  L-2  are  said  to  be  electrostatically  coupled  through 
condensers  C-l  and  C-2. 

the  same  way  as  in  systems  having  a  strictly  closed  circuit.  But  owing 
to  the  increased  voltage  applied  to  the  detector,  amplification  of  the 
incoming  signal  is  secured. 


Vacuum  Tubes  In  Wireless  Communication 


11 


(a)  The  Necessity  for  Tuning.  It  is  'essential  for  best  results  in 
any  two  circuit  receiving  system,  that  the  open  and  closed  oscillation 
circuits  (or  the  antenna  and  detector  circuits)  be  tuned  to  exact  reso- 
nance, for  it  is  under  this  condition  only,  that  the  oscillating  electro- 
motive force  impressed  across  the  terminals  of  the  detector  reaches  its 


-=--<•'  Figure  9 

Showing  the  use  of  open  circuit  oscillators  in  wireless  receiving 
systems.  The  circuit  from  detector  D  through  coil  L-2  and  coil  L-l  to 
E  is  tuned  to  the  frequency  of  the  incoming  oscillations  or  adjusted 
until  maximum  potential  exists  at  the  free  end  of  L-2. 


maximum  value.  If  the  energy  consumption  of  the  detector  is  large, 
less  exactness  of  tuning  may  give  good  results,  but  approximate  reso- 
nance is  required  in  all  cases  unless  the  distance  is  short  or  the  received 
energy  very  large. 

To  tune  an  oscillation  circuit  to  a  given  impressed  frequency,  it  is 
necessary  to  reduce  its  reactance  to  zero  for  the  frequency  of  the  in- 
coming signal.  This  is  substantially  what  the  operator  at  the  receiving 
station  does  when  he  adjusts  the  receiver  for  maximum  strength  of  sig- 
nals, that  is,  he  makes  the  reactance  of  the  capacity  equal  to  the  react- 
ance of  the  inductance  for  a  given  impressed  frequency  of  oscillation. 
The  amplitude  of  the  receiver  current  is  then  governed  solely  by  the 
impressed  electromotive  force  and  the  total  equivalent  resistance  of  the 
circuit  (including  all  losses). 

Such  tuning  is  accomplished  by  variable  inductances  and  variable 
condensers.  A  variable  condenser  may  be  of  the  interleaved  flat  type 
shown  in  Figure  10,  or  of  the  tubular  type  shown  in  Figure  11.  The 
majority  have  air  dielectric.  In  one  type  the  plates  are  separated  by 
thin  sheets  of  rubber.  The  capacity  of  a  variable  condenser  for  receiv- 
ing purposes  rarely  exceeds  .01  microfarad,  more  common  values  being 
.0005  to  .005  microfarads. 


12 


Vacuum  Tubes  In  Wireless  Communication 


Tuning  inductances  are  generally  of  the  single  layer  type  varying 
from  one  micro-henry  to  three  or  four  milli-henries  inductance.  Multi- 
layered  coils  have  found  some  use,  but  their  self-capacity  between  layers 


Figure  9a 


Diagrammatic  sketch  of  the  complete  radio  frequency  circuits  of  the  trans- 
mitter and  receiver  in  a  wireless  system.  The  closed  oscillation  circuit  of  a 
transmitter  is  represented  by  the  condenser  C,  the  primary  coil  of  the  oscillation 
transformer  L-l,  and  the  spark  gap  G.  The  secondary  or  antenna  circuit  com- 
prises the  aerial  AI,  the  secondary  coil  L-2,  the  antenna  loading  inductance  L-3, 
the  short  wave  condenser  C-l,  and  the  aerial  ammeter  A. 

The  circuit  of  the  receiving  apparatus  is  designated  by  the  antenna  wires 
A",  the  antenna  loading  inductance  L-4,  the  primary  winding  of  the  receiving 
transformer  L-5,  and  the  short  wave  variable  condenser  C-2.  The  secondary 
or  closed  circuit  includes  the  secondary  coil  L-6,  the  shunt  variable  condenser 
C-3,  the  rectifier  D,  the  head  telephone  P,  and  the  shunt  telephone  condenser  C-4. 

The  condenser  C  of  the  transmitter  is  generally  charged  1000  times  per 
second  by  a  500-cycle  alternator  and  transformer.  1000  sparks  discharge  across 
the  gap  G  per  second  and  1000  groups  of  radio  frequency  oscillations  are  re- 
leased in  the  closed  circuit  C,  L-l,  G.  Coil  L-l  acts  inductively  upon  coil  L-2  and 
groups  of  oscillations  of  similar  frequency  flow  in  the  antenna  circuit  AI,  L-3, 
L-2,  C-l,  A,  E,  provided  this  circuit  is  tuned  to  resonance.  As  the  transmitter 
antenna  oscillates,  it  radiates  an  electromagnetic  wave  motion  which  acts  upon 
the  receiving  antenna  circuit  A",  L-4,  L-5,  C-2,  E,  which  is  carefully  tuned  to 
resonance  with  the  transmitter.  The  coil  L-5  acts  inductively  upon  L-6,  the 
circuit  L-6,  C-3  being  tuned  to  resonance  by  means  of  condenser  C-3.  Oscillations 
of  the  transmitter  frequency  flow  in  the  closed  circuit  where  they  are  rectified 
by  detector  D,  and  made  audible  in  the  head  telephone  P. 

This  diagram  is  representative  of  wireless  systems  for  the  production  of 
damped  oscillations.  It  is  to  be  observed  that  four  circuits  require  tuning, 
namely,  the  closed  and  open  circuits  of  the  transmitter,  and  the  closed  and  open 
circuits  of  the  receiver.  All  are  tuned  substantially  to  the  same  frequency 
of  oscillation. 


Vacuum  Tubes  In  Wireless  Communication 


13 


is  found  objectionable  at  the  higher  frequencies.     They  have,  however, 
been  employed  with  good  results  at  longer  wave  lengths. 

It  is  now  quite  common  to  wind  the  primary  and  secondary  coils  of 
a  receiving  tuner  in  two  or  more  layers.    A  maximum  of  inductance  is 


STATIONARY 


CONTROL  KNOB 


MOVA8LL   PUTES 


STATIONARY  PUTES 


Figure  10 

A  variable  condenser  of  the  multiple  plate  type  as  used  in  tuning 
receiving  circuits. 


JNSULATION 


OUTER  TUBE 


POSITION   OF  MAXIMUM  CAPACITY 


Figure  11 
A  variable  condenser  of  the  tubular  type. 

thus  obtained  with  a  minimum  of  resistance.  A  multi-point  inductance 
for  tuning  purposes  is  shown  in  Figure  12.  In  this  diagram,  the  switch 
marked  "Units"  cuts  in-  single  turns  in  the  circuit,  and  the  switch 
marked  "Tens"  cuts  in  ten  turns  at  each  contact  point. 


14 


Vacuum  Tubes  In  Wireless  Communication 


(b)  Tuning  in  Practice.  To  adjust  a  receiver  for  maximum  response 
from  a  given  transmitter,  the  operator  first  tunes  the. closed  or  secondary 
circuit  to  the  desired  wave  length,  and  then  couples  closely  thereto  the 
antenna  or  primary  coil.  He  then  increases  or  decreases  the  inductance 
or  capacity  or  both  simultaneously,  in  the  antenna  circuit  until  the 


IpOOOOOpOOOOOOOOMOMOMjHMiMilfliM!"' 


Figure  12 

Showing  how  the  inductance  of  a  radio  frequency  coil  may  be  varied 
from  one  turn  to  maximum  through  the  use  of  two  multi-point  switches. 
In  this  diagram  the  switch  marked  "Units"  cuts  in  single  turns,  and  the 
switch  marked  "Tens,"  ten  turns  in  a  group.  For  example,  if  one  turn 
is  cut  in  at  the  "Units"  switch  and  ten  turns  at  the  "Tens"  switch,  the 
circuit  will  include  eleven  turns. 

desired  signal  is  heard.  If  the  receiving  apparatus  has  been  calibrated 
by  a  wave  meter,  all  the  necessary  tuning  can  be  done  before  the  trans- 
mitting station  begins  operation. 

•Selectivity  in  wireless  receiving  apparatus,  that  is,  discrimination 
between  stations,  is  secured: 

(1)  by  loose  coupling  of  the  primary  and  secondary  circuits; 

(2)  by  the  use  of  large  values  of  inductance  at  the  base  of  the  aerial 

for  a  given  wave  length; 

(3)  by  judicious  use  of  the  shunt  secondary  condenser. 

The  tuner  may  be  adjusted  for  broad  tuning  (response  from  several 
transmitters  simultaneously)  by  closely  .coupling  the  primary  and  second- 
ary circuits.  This  increases  the  natural  damping  of  the  system  and  the 
complete  circuit  will  therefore  respond  over  a  range  of  frequencies  with- 
out requiring  accurate  adjustment.  On  the  other  hand,  loose  coupling 
of  the  open  and  closed  circuits  permits  sharp  tuning,  that  is,  the  receiver 
responds  with  vigor  to  oscillations  of  a  single  frequency,  and  the  more 
exact  the  resonance  the  better  the  signal. 


Vacuum  Tubes  In  Wireless  Communication 


15 


4.  THE  TELEPHONE  RECEIVER.— The  simple  magnetic  tele- 
phone receiver  is  universally  employed  to  detect  the  currents  induced 
in  the  wireless  receiving  aerial  by  the  distant  transmitter.  Certain 
phenomena  connected  with  its  operation  have  been  touched  upon  in 
paragraph  1,  but  will  now  be  considered  more  in  detail. 


Figure  12a 

Front  view  of  the  American  Marconi  Co.'s  type  106  receiving  tuner. 
All  tuning  apparatus  is  mounted  on  the  front  of  the  panel  board  con- 
venient to  the  operator. 

This  tuner  is  of  the  inductively  coupled  type  and  is  fitted  with  a 
carborundum  detector.  The  wave  length  is  variable  between  200  and 
3,500  meters. 


END  TURN 
SWITCH  " 


POTEMT10METER 


SECONDARY 


SUI1ER 


PRIMARY 


SHORT  WAVE 

CONDENSER 


Figure  12b 

Rear  view  of  the  type  106  tuner,  showing  the  tuning  transformer, 
variable  condensers,  potentiometer,  and  buzzer  tester. 


16 


Vacuum  Tubes  In  Wireless  Communication 


In  the  diagram  of  Figure  13,  the  bobbins  of  the  telephone  magnet 
windings  are  indicated  at  B-l  and  B-2,  the  cores  of  which  are  perma- 
nently magnetized  by  the  horseshoe  or  ring  magnet  M. 

If  direct  current  flows  between  A  and  B  in  one  direction  it  strength- 
ens the  total  magnetic  field  and  D  is  drawn  down  further,  but  if  the 
direction  of  the  current  is  then  reversed,  the  total  field  acting  on  D  is 
decreased  and  the  diaphragm  is  released  by  an  amount  depending  upon 
the  demagnetizing  effect  of  the  current. 


A  B 

Figure  13 

Showing    the    fundamental    construction    of    the 
telephone  receiver. 

If  an  alternating  current  passes  through  the  receiver  winding, 
similar  effects  are  observed,  that  is,  a  cycle  of  current  will  cause  the 
diaghragm  to  go  through  the  movement  outlined  in  the  preceding  para- 
graph. 

It  is  now  quite  clear  that  if  we  desire  to  set  the  diaphragm  into 
motion  or  produce  sound,  we  must  supply  to  the  telephone  a  current 
that  varies  in  amplitude.  This  current  may  be  a  fluctuating  direct  cur- 
rent, a  sinusoidal  alternating  or  a  complex  alternating  current.  It  may 
be  said  in  general  that  the  receiver  diaphragm  repeats  the  variations  of 
any  variable  E.M.F.  but  not  always  without  distortion. 

Beyond  this,  currents  of  a  definite  frequency  will  produce  more 
sound  with  a  given  minimum  E.M.F.  than  those  of  other  frequencies. 
The  fact  is  that  the  diaphragm  of  a  telephone  receiver  possesses  a  "natu- 
ral frequency  of  vibration, ' '  that  is,  if  deflected  to  one  side  momentarily 
and  released,  the  diaphragm  will  vibrate  at  a  rate  determined  by  its 
mass  and  elasticity. 

If  the  receiver  is  actuated  by  an  alternating  current  of  sinusoidal 


Vacuum  Tubes  In  Wireless  Communication 


17 


form,  and  of  variable  frequency,  the  diaphragm  will  vibrate  most  strongly 
when  the  impressed  frequency  equals  the  natural  frequency  of  the 
diaphragm.  This,  for  the  average  telephone  used  in  wireless  telegraphy 
is  near  to  1000  cycles  per  second,  but,  of  course,  it  varies  with  the  con- 
struction of  the  receiver. 


CONTACT 
STUD5 


CONTACT 
SEGMENTS 


BRUSHES 


Figure  13a 

Showing  the  end  turn  switch  employed  in  the  primary  and  second- 
ary circuits  of  the  Marconi  type  106  tuner.     This  switch  divides  the 
tuning  coils  into  groups  to  eliminate  end  turn  losses. 

The  important  point  to  be  brought  out  here  is  that  when  the  fre- 
quency of  the  applied  current  is  such  as  to  impulse  the  diaphragm  more 
than  1000  times  per  second,  its  motion  as  the  frequency  is  increased 


18  Vacuum  Tubes  In  Wireless  Communication 

becomes  less  and  less  audible  for  a  given  E.M.F.  until  at  frequencies 
above  20,000  cycles  per  second  no  sound  appreciable  to  the  human  ear 
is  obtained. 

In  the  receiving  circuits  of  commercial  wireless  telegraphy,  as  we 
have  already  mentioned,  alternating  currents  flow  at  various  frequencies 
between  20,000  and  1,000,000  cycles  per  second;  hence,  we  see  at  once 
the  futility  of  using  the  telephone  receiver  as  a  sound  indicator  of  radio 
frequency  oscillations. 

Then  we  can  obtain  sound  in  the  telephone  from  incoming  radio 
frequency  currents  only  by  employing  a  device  which  will  convert  such 
currents  into  some  form  of  an  audio  frequency  current  that  will  give 
a  response  in  the  head  telephone. 

Just  how  this  is  accomplished  in  the  Fleming  oscillation  valve  will 
be  described  in  the  chapter  following. 


PART  1 

THE  OSCILLATION   VALVE  IN   RADIO 
TELEGRAPHY 

5.  FOREWORD. — The  student  about  to  take  up  the  profession  of 
a  commercial  or  government  wireless  operator  often  is  confused  by  the 
seeming  complication  of  circuits  encountered  in  connection  with  the 
operation  of  the  vacuum  valve.  Moreover,  he  is  confronted  with  a  new 
terminology  and  a  glittering  array  of  trade  names  for  vacuum  tubes  of 
different  construction. 

Although  the  oscillation  valves  employed  in  wireless  telegraphy  are 
known  in  the  art  under  various  trade  appellations,  the  author  will  employ 
the  terms  "vacuum  valve/'  "vacuum  bulb/'  "vacuum  tube,"  or 
"vacuum  tube  detector"  interchangeably. 

It  is  not  the  purpose  of  this  volume  to  discuss  the  relative  merits 
of  the  various  types  of  vacuum  tubes,  or  to  go  deeply  into  their  con- 
struction, but  an  effort  will  be  made  to  present  in  simple  language  the 
conclusions  arrived  at  by  the  inventor  of  the  oscillation  valve,  Dr.  J.  A. 
Fleming,  and  by  various  investigators  of  prominence,  among  whom  may 
be  mentioned  Captain  Edwin  H.  Armstrong,  Roy  A.  Weagant,  and  Dr. 
Irving  Langumuir.  A  further  object  is  to  show  the  radio  operator  the 
circuits  and  functioning  of  the  valve  in  practice. 

In  order  to  present  the  subject  in  a  simple  manner,  the  author  has 
deviated  slightly  from  strict  scientific  terminology  and  mode  of  presenta- 
tion, but  now  that  the  motive  for  r  doing  is  made  known,  due  allowance 
will  be  made  by  the  expert  engineer. 

It  would  be  well  to  keep  in  mind  that  the  vacuum  tube  can  be 
employed  to  detect  the  passage  of  high  frequency  currents  through  the 
medium  of  a  telephone,  or  it  may  act  as  a  generator  of  radio  frequency 
currents  for  the  purpose  of  wireless  communication.  Both  uses  will  be 
treated  but  the  text  will  bear  particularly  on  the  use  of  the  valve  as  an 
oscillation  detector  in  wireless  telegraphy. 

19 


20  Vacuum  Tubes  In  Wireless  Communication 

6.  WHAT  IS  AN  ELECTRO N?-The  vacuum  tube  when  used  as 
an  oscillation  detector  in  radio  is  frequently  called  an  Electron  Relay 
and  the  term  implies  that  electrons  are  useful  agents  in  its  operation; 
hence  the  question  naturally  arises  in  the  mind  of  the  student,  "What 
is  an  electron?"  Briefly,  we  may  state  that  the  electron  is  the  smallest 
subdivision  of  matter  which  mankind  recognizes  carrying  the  smallest 
known  charge  of  negative  electricity. 

It  has  been  held  by  science  for  years  that  matter  is  built  up  of 
distinct  particles  or  units  termed  atoms  and  molecules.  The  molecule 
first  was  assumed  to  be  the  smallest  quantity  of  matter  that  can  have  a 
separate  existence  or  take  part  in  chemical  action,  but  more  rigorous 
research  points  to  the  fact  that  the  molecule  is  made  up  of  smaller  ele- 
ments which  are  termed  atoms;  that  is,  a  molecule  may  be  composed  of 
several  atoms.  The  atom,  then,  was  assumed  to  be  the  very  smallest 
quantity  of  an  element  that  can  exist,  but  latter  day  researches  have 
revealed  that  the  atom  may  be  further  subdivided  into  corpuscles  or 
particles  termed  electrons. 

The  apparent  mass  of  an  electron  is  about  one  eighteen-hundredth 
part  of  that  of  an  atom  of  hydrogen,  which  is  the  smallest  of  the  chemical 
atoms. 

According  to  the  theory  now  in  vogue  concerning  the  ultimate  nature 
of  matter,  an  atom  consists  of  a  definite  number  of  electrons  grouped 
about  a  nucleus  having  a  positive  charge,  and  so  long  as  none  of  the 
component  electrons  are  driven  from  the  atom,  the  latter  possess  no 
detectable  electrical  charge. 

But  let  an  electron  be  detached  from  the  atom,  then  the  atom  be- 
comes what  is  known  as  a  "positive  ion"  and  it  exhibits  the  properties 
of  a  positively  charged  body. 

On  the  other  hand,  if  an  electron  is  added  to  a  normal  uncharged 
atom,  the  latter,  in  this  state,  possesses  a  negative  charge  and  is  termed 
a  "negative  ion."  In  summary,  the  positive  ion  possesses  a  deficiency 
of  electrons  and  the  negative  ion  an  excess  of  electrons. 

Although  a  number  of  congregated  electrons  go  ultimately  to  make 
up  a  particle  of  matter,  we  should  not  conceive  of  electrons  in  terms  of 
matter  as  we  ordinarily  understand  this  term  in  solids.  Rather  must 
we  think  of  electrons  in  terms  of  electricity,  and  in  this  way  we  can 
comprehend  in  some  measure  the  extreme  mobility  which  they  possess 
as  compared  to  "solid  bodies."* 

In  the  original  sense   of   expressing   "positive"   and   "negative" 


*For  an  interesting  article  on  modern  conceptions  of  the  electron  see  the 
paper,  "Modern  Physics,"  Proceedings  of  the  American  Institute  of  Electrical 
Engineers. 


Vacuum  Tubes  In  Wireless  Communication  21 

charges  of  electricity  an  electron  constitutes  a  negative  charge.  In  fact, 
when  a  number  of  electrons  are  detached  from  any  atom  we  say  that 
there  has  been  a  movement  of  negative  electricity,  and  we  should  now 
accept  the  statement  that  the  flow  of  electricity  is  a  movement  of 
electrons. 

Electrons  may  act  as  carriers  of  electricity  between  two  conductors 
separated  in  vacuua.  Some  of  the  present  theories  as  to  how  they  are 
made  use  of  in  the  vacuum  tube  oscillation  detectors  of  radio  telegraphy 
we  shall  describe  and  explain  further  on. 

7.  HOW  MAY  ELECTRONS  BE  SET  INTO  MOTION?— We 

have  stated  in  effect  that  in  order  to  liberate  electrons  we  must  disrupt 
an  atom.  Of  the  various  methods  for  disrupting  an  atom  the  one  em- 
ployed in  the  vacuum  valve  tubes  will  be  given  consideration. 

It  has  been  known  for  many  years  that  the  space  about  a  heated 
metal  is  a  conductor  of  electricity.  It  has  been  demonstrated  more 
recently  that  it  is  due  to  the  release  of  electrons  and  that  if  an  incan- 
descent metal  is  placed  in  a  bulb  exhausted  of  air  or  gases,  pure  electrons 
are  liberated  from  the  metal.  This  is  the  most  convenient  method  for 
driving  electrons  from  a  metallic  body  for  use  in  connection  with  oscilla- 
tion detectors,  and  as  stated  before,  the  other  methods  for  doing  this  will 
not  be  discussed. 

For  any  useful  result  the  vacuum  employed  must  be  of  a  high  order, 
approximating  that  of  an  incandescent  lamp  or  better,  as  the  presence 
of  any  considerable  quantity  of  gas  either  prevents  or  greatly  interferes 
with  a  useful  action. 

8.  FLEMING'S  DISCOVERY.— The  first  to  employ  the  emission 
of  electrons  from  heated  metals,  to  rectify  or  to  detect  radio  frequency 
oscillations  in  a  wireless  telegraph  receiving  system,  was  Dr.  J.  A.  Flem- 
ing, of  London,  England.    He  named  his  product  a  glow  lamp  oscillation 
detector  or  oscillation  valve,  the  latter  name  being  adopted  because  he 
found  that  the  lamp  would  conduct  electricity  better  in  one  direction 
than  in  the  opposite  direction.     Hence,  if  the  valve  was  connected  in 
series  with  a  source  of  radio  frequency  oscillations,  one-half  of  each  cycle 
was  suppressed,  the  circuit  being  traversed  by  a  pulsating  direct  current. 
The  flow  of  such  currents  could  then  be  detected  by  a  telephone  receiver 
or  a  sensitive  direct  current  instrument  such  as  the  galvanometer.    In 
more  scientific  terms  the  glow  lamp  was  found  to  possess  unilateral  con- 
ductivity or  assymetric  resistance. 

This  discovery  marked  a  most  important  step  in  the  progress  of  the 
radio  art.  Immediately  the  results  of  Fleming's  investigations  were 
published,  intense  interest  was  aroused  throughout  the  scientific  world, 


22 


Vacuum  Tubes  In  Wireless  Communication 


and  simultaneously  a  steady  commercial  application  of  the  vacuum  valve 
as  a  detector  of  electric  waves  took  place. 

9.  THE  OSCILLATION  VALVE.— One  form  of  Fleming's  oscil- 
lation valve  for  use  as  an  oscillation  detector  in  wireless  telegraphy  is 
shown  in  Figure  14. 

A  carbon  filament  F  brought  to  incandescence  by  a  small  storage 
battery  is  surrounded  by  a  metallic  cylinder  P  such  as  copper  or  nickel. 
Later  types  of  Fleming's  valves  employed  a  tungsten  filament  and  a 
copper  cylinder,  or  in  place  of  the  latter,  a  small  metallic  plate. 


Figure    14 
Fleming's  oscillation  valve  in  a  ?':.%ple  form. 

In  order  to  secure  the  valve  effect  the  terminals  of  the  filament  first 
must  be  connected  to  a  4  or  6  volt  battery,  the  degree  of  incandescence 
being  carefully  adjusted  by  a  series  rheostat.  The  filament  then  emits 
electrons.  The  current  to  be  rectified,  is  next  impressed  upon  the  termi- 
nals C,  D,  that  is,  C,  D,  are  connected  in  series  with  the  circuit  in  which 
the  valve  effect  is  desired. 

Certain  phenomena  bearing  upon  the  action  of  the  valve  as  an  oscil- 
lation detector  are  summed  up  in  the  following  statements : 

(1)  The  filament  F  when  heated  emits  electrons; 

(2)  These  electrons  can  be  drawn  to  the  cylinder  pr  plate  P  by  charg- 

ing the  plate  to  a  positive  potential; 

(3)  When  electrons  are  drawn  to  P,  the  vacuous  space  between  the 

filament  and  the  plate  is  conductive  in  one  direction  only,  i.  e., 
a  battery  current,  for  instance,  can  flow  from  the  plate  P  to 
filament  F,  but  not  in  the  opposite  direction; 

(4)  The  amount  of  current  that  can  be  passed  from  the  plate  to  fila- 

ment is  definitely  limited  for  a  given  set  of  conditions. 


Vacuum  Tubes  In  Wireless  Communication 


23 


The  student  should  give  this  preliminary  outline  of  action  careful 
study. 

Now  any  device  which  will  pass  a  current  of  electricity  in  one  direc- 
tion and  will  wholly  or  partially  obstruct  the  flow  in  the  opposite  direc- 
tion may  be  termed  a  "  rectifier, "  because  if  it  is  connected  in  the  path 
of  an  alternating  current,  it  will  suppress  one-half  of  each  cycle  and 
therefore  the  circuit  will  be  traversed  by  a  pulsating  direct  current. 

A  rectifier  also  is  said  to  possess  "unilateral  conductivity,"  meaning, 
of  course,  that  it  will  conduct  electricity  in  one  direction  only.  Its  ability 
to  rectify  currents  of  extremely  high  frequency  is  one  of  the  important 
properties  of  the  oscillation  valve.  The  part  which  the  rectifier  plays  in 
the  detection  of  wireless  signals  has  been  mentioned  in  paragraph  2,  but 
it  will  be  treated  more  in  detail  in  Part  II.  The  electron  current  in  the 
valve  will  be  given  the  first  consideration. 

10.  DEMONSTRATION  OF  RECTIFYING  PROPERTIES.— 

The  rectifying  properties  of  the  vacuum  tube  can  be  demonstrated  by 
the  apparatus  indicated  in  the  diagram  of  Figure  15.  Here,  filament  F 
is  lit  to  incandescence  by  battery  5-1  of  4  to  12  volts.  An  E.  M.  F. 
variable  up  to  100  volts  or  more  is  applied  between  F  and  P  by  battery 
5-2.  A  milliammeter  shown  at  A  is  connected  in  series  with  B-2.  As 


Figure  15 

Apparatus   for   demonstrating   the   rectifying   properties   of  the 
two-electrode  oscillation  valve. 

stated  under  (2),  paragraph  9,  it  is  by  thus  charging  plate  P  to  a  posi- 
tive potential  that  electrons  are  drawn  over  from  the  filament. 

In  this  experiment,  meter  A  will  register  only  when  the  positive 
terminal  of  battery  B-2  is  connected  to  the  plate.  If  connected  in  the 
opposite  way,  little  or  no  current  will  flow.  Hence,  if  an  alternating 
E.  M.  F.  is  impressed  across  F  and  P,  current  can  flow  from  P  to  F  only 


24  Vacuum  Tubes  In  Wireless  Communication 

when  P  is  charged  positively.  The  tube  thus  becomes  a  rectifier  of 
alternating  currents,  and  will  perform  this  function  at  any  frequency 
up  to  several  million  cycles  per  second. 

Now,  merely  as  a  matter  of  illustration,  we  have  represented  (in 
Figure  15)  the  passage  of  the  electrons  which  are  attracted  to  the  plate 
P  from  filament  F  by  double  pointed  arrows  and  the  current  supplied 
by  battery  B-2  which  flows  from  plate  P  to  filament  F  by  single  pointed 
arrows.  At  first  sight  there  may  appear  to  be  two  forces  acting  against 
each  other,  but  the  seeming  contradiction  can  easily  be  explained. 

The  assumption  that  electricity  flows  from  a  point  of  positive  poten- 
tial to  one  of  negative  potential  was  made  and  accepted  by  scientists 
many  years  before  there  was  any  knowledge  as  to  what  a  flow  of  electricity 
is,  but  since  the  introduction  of  the  electron  theory  (keep  in  mind  the 
fact  that  electrons  are  negative  charges  of  electricity)  it  is  necessarily 
assumed  that  the  transfer  of  electricity  takes  place  in  the  direction  from 
the  negatively  charged  ~body  to  the  positively  charged  body. 

It  will  prevent  confusion  at  the  outset,  then,  if  the  student  under- 
stands that  in  conformation  with  the  old  theory  electricity  flows  in  the 
direction  opposite  to  the  flow  of  the  electrons. 

Now,  to  withdraw  electrons  from  the  filament  F  to  the  plate,  the 
latter  must  be  charged  to  a  positive  potenial.  Thus,  it  is  evident  that 
electrons  flow  from  the  filament  to  the  plate  by  virtue  of  the  electrostatic 
field  maintained  by  the  battery  B-2.  The  strength  of  this  field,  of  course, 
varies  with  the  E.  M.  F.  of  B-2.  If,  on  the  other  hand,  P  is  charged 
negatively,  the  electrons  are  driven  back  to  the  filament  and  the  current 
will  be  reduced  to  zero. 

11.  NON-UNIFORM  CONDUCTIVITY  OF  THE  VALVE.— 

The  rectifying  properties  of  the  tube  have  been  treated  in  the  preceding 
paragraph.  With  the  connections  of  Figure  15,  it  can  also  be  shown  that 
the  conductivity  of  the  space  between  P  and  F  is  not  a  constant  in  the 
direction  in  which  it  conducts  more  freely,  i.  e.,  the  apparent  resistance 
varies  with  the  applied  E.  M.  F. 

More  clearly  if  the  valve  is  placed  in  series  with  a  source  of  current 
the  complete  circuit  will  not  obey  Ohm's  Law.  Just  as  in  the  case  of 
the  carborundum  rectifier,*  if  battery  B-2  of  Figure  15  is  shunted  by  a 
potentiometer,  and  the  voltage  is  steadily  increased  from  a  small  value 
to  some  upper  limit  of  the  battery,  the  readings  of  the  meter  A  will  not 
accord  with  Ohm's  Law. 

During  the  first  increase  in  voltage,  the  reading  of  the  current  will 
be  low,  indicating  a  high  value  of  resistance,  and  after  a  more  or  less 


*See  pages  137,  138,  139  author's  "Practical  Wireless  Telegraphy." 


Vacuum  Tubes  In  Wireless  Communication 


25 


critical  point  is  passed,  the  current  (in  micro-  or  milli-amperes)  will 
rapidly  increase,  indicating  a  much  lower  resistance.  The  current- voltage 
readings  thus  obtained  can  be  plotted  in  the  form  of  a  curve  as  in  Figure 
16.  Up  to  the  point  A  on  the  curve  an  increase  of  E.  M.  F.  of  the  bat- 


I 


5.5 

o 

cc 


I 

o 


CM 


E-2 


E-3 


10 


50 


E-4 
60 


E-5 
70 


E-l 

10  30  40 

E.  M.  F.  OF    B-2 

Figure  16 

Characteristic  curve  of  two-electrode  Fleming  valve,  indicating  the 
strength  of  the  electron  current  with  different  values  of  plate  potential. 

tery  B-2  results  only  in  a  slight  increase  of  current  as  shown  by  the 
readings  on  the  vertical  axis.  But  if  the  voltage  be  increased  from  E-l 
to  E-3  or  E-4,  the  current  in  the  plate  circuit  rises  very  rapidly.  From 
point  C  on,  there  will  be  no  further  increase  of  current  with  increase 
of  the  plate  E.  M.  F.  At  this  point  the  valve  is  said  to  be  saturated. 


26  Vacuum  Tubes  In  Wireless  Communication 

If  the  data  obtained  in  this  experiment,  plotted  on  cross-section 
paper,  gave  a  straight  line  instead  of  a  curve,  then  Ohm's  Law  would 
be  obeyed,  and  the  valve  would  possess  uniform  conductivity,  but  since 
the  curve  shows  that  it  does  not  obey  Ohm's  Law,  the  valve  possesses 
non-uniform  conductivity. 

The  curve  of  Figure  16  is  then  called  the  "characteristic"  curve  of 
the  valve  and  the  adjustment  of  the  plate  current  to  the  so-called  critical 
point  on  the  curve  during  the  reception  of  signals  renders  it,  through 
the  medium  of  a  telephone,  a  sensitive  indicator  of  feeble  radio  frequency 
currents  such  as  flow  in  the  receiving  circuits  of  a  wireless  telegraph 
system. 

The  critical  point  on  the  curve  is  usually  on  the  lower  bend  but 
response  can  be  secured  at  the  upper  bend  as  well ;  or  under  certain  con- 
ditions, other  points  on  the  characteristic  curve  may  be  employed  in  radio 
reception.  This  will  be  treated  more  in  detail  further  on. 

12.    HOW    THE    VALVE    BECOMES    SATURATED.  —  The 

strength  of  the  electron  current  flowing  from  F  to  P  is  limited  in  two 
ways:  First,  by  the  temperature  limitation  of  the  filament,  and  second, 
by  the  so-called  space  charge.  At  point  C  in  the  curve  of  Figure  16,  the 
increase  of  current  through  the  valve  falls  off  rapidly.  Any  further 
increase  in  E.  M.  F.  of  B-2  gives  no  further  increase  of  current.  At  this 
point  the  valve  is  saturated. 

It  would  be  well  at  this  juncture  for  the  student  to  understand  that 
the  amount  of  current  passed  per  second  from  plate  to  filament  varies 
directly  with  the  supply  of  electrons.  In  fact,  it  is  assumed  that  1019 
electrons  passing  from  F  to  P  per  second  are  the  equivalent  of  one 
ampere.  As  mentioned  above,  the  first  limitation  is  found  in  the  supply 
of  electrons  given  off  by  the  filament,  which  varies  with  the  temperature. 
Only  the  equivalent  current  of  this  supply  of  electrons  can  flow  no  mat- 
ter how  much  we  increase  the  E.  M.  F.  of  B-2. 

Then,  to  increase  the  electronic  emission,  we  must  increase  the  tem- 
perature of  the  filament  provided  the  maximum  permissible  temperature 
has  not  been  reached.  If  the  E.  M.  F.  of  B-2  be  then  increased  a  new 
curve  A,  B,  D  (Figure  16)  will  be  obtained,  i.  e.,  the  electron  current 
from  F  to  P  will  increase.  Further  increase  of  the  E.  M.  F.  of  B-2  gives 
another  point  of  saturation  D,  and  no  matter  how  much  the  E.M.F. 
of  B-2  is  raised  no  increase  of  current  will  result  because  all  of  the 
electrons  available  have  been  drawn  to  the  plate. 

We  might  state  this  in  another  way  by  saying  that  the  electrons  are 
attracted  to  the  plate  by  the  electrostatic  field  (between  the  plate  and 
filament)  maintained  by  battery  B-2,  and  if  the  temperature  of  the  fila- 


Vacuum  Tubes  In  Wireless  Communication 


27 


ment  is  fixed  we  need  only  increase  the  voltage  of  B-2  to  that  value  neces- 
sary to  draw  over  all  the  electrons  given  off  by  the  filament,  if  we  want 
the  maximum  current  to  flow.  Further  increase  of  the  E.  M.  F.  of  B-2, 
as  stated  before,  will  not  increase  the  electron  current  because  no  more 


Z 

UJ 

CC 

at 


< 
a. 


.25 


.50  .75  1.0 

FILAMENT      CURRENT 


I.Z5 


1.50 


Figure  17 

Characteristic  curve  of  two-electrode  valve  showing  how  the  plate 
current  varies  with  different  degrees  of  filament  temperature,  the  po- 
tential of  the  plate  being  held  constant. 


28  Vacuum  Tubes  In  Wireless  Communication 

electrons  are  available.     Thus  we  see  that  the  plate  current  is  limited 
by  the  temperature  of  the  filament. 

In  addition  to  the  restricted  plate  current  due  to  the  filament  tem- 
perature limitation  mentioned  above,  Dr.  Langumuir  declares*  that  the 
plate  current  is  limited  for  a  given  plate  voltage  by  reason  of  the  space 
charge  within  the  bulb.  He  remarks  that  the  electrons  flowing  between 
the  filament  and  plate  constitute  a  negative  electric  charge  in  space 
which  repels  the  electrons  escaping  from  the  filament,  causing  some  of 
them  to  return  to  the  filament;  that  is,  only  a  part  of  the  electrons 
emitted  by  the  filament  reach  the  plate,  the  remainder  being  repelled 
by  the  electrons  in  space  and  they  therefore  return  to  their  source. 

More  clearly,  if  the  plate  voltage  is  fixed  and  the  filament  tempera- 
ture is  steadily  increased,  a  condition  will  be  reached  where  an  increase 
of  filament  temperature  will  not  result  in  an  increase  of  the  plate  current 
on  account  of  the  space  charge.  Then,  to  increase  the  plate  current  it 
becomes  necessary  to  increase  the  voltage  of  the  plate  battery. 

The  space  charge  can  also  be  diminished  by  altering  the  construction 
of  the  bulb,  i.  e.}  by  bringing  the  plate  and  filament  closer. 

Another  curve,  Figure  17,  will  serve  to  make  clear  the  relation  be- 
tween a  progressive  increase  of  current  through  the  filament  with  a 
definite  value  of  E.M.F.  at  B-2  and  the  corresponding  current  in  the  plate 
circuit. 

If  the  temperature  of  the  filament  is  progressively  increased,  then 
a  curve  A,  D,  E,  is  obtained.  The  curve  indicates  that  at  point  E,  the 
space  charge  tends  to  neutralize  the  electron  current.  In  other  words, 
the  plate  current  increases  with  increase  of  filament  temperature  from  A 
to  G,  but  falls  off  rapidly  from  G  to  C. 

If  the  filament  temperature  exceeds  A,  G,  and  the  E.M.F.  of  B-2  is 
increased,  a  new  curve  A,  D,  F,  is  obtained,  which  indicates  the  flow  of 
an  increased  current  through  the  plate  circuit.  It  is  clear  from  Figure 
17  that  if  the  filament  current  is  less  than  A,  G,  the  current  in  the  plate 
circuit  will  not  increase  with  increase  of  the  E.M.F.  of  B-2,  Figure  15. 


^Proceedings   of  the  Institute  of  Radio  Engineers,   Sept.,   1915,  pages  268 
and  274. 


PART  II 

PRACTICAL     APPLICATIONS    OF    THE 
OSCILLATION    VALVE 

13.  IN  GENERAL. — We  have  established  four  important  facts  con- 
cerning the  vacuum  tube  and  the  telephone  receiver: 

(1)  The  tube  is  conductive  in  one  direction  and  therefore  may  be 

employed  as  a  rectifier  of  alternating  currents;  i.  e.,  it  pos- 
sesses assymetrical  conductivity. 

(2)  A   characteristic   curve   of  the   tube   shows  that  it  does  not,  in 

respect  to  conduction,  obey  Ohm's  law;  i.  e.,  its  resistance 
varies  with  the  applied  E.  M.  F. 

(3)  The  telephone  receiver  will  not  give  audible  response  to  radio 

frequency  currents  (currents  in  excess  of  10,000  cycles  per 
second). 

(4)  By  means  of  a  rectifier,  radio  frequency  currents  occurring  in  audio 

frequent  groups  (damped  oscillations)  can  be  converted  to 
decaying  direct  currents  which,  if  they  flow  through  the  tele- 
phone, will  give  audible  response.  (See  paragraphs  2  and  9.) 

These  four  statements  will  assist  the  student  to  understand  how  the 
incoming  radio  frequency  oscillations  in  a  wireless  receiving  system  are 
translated  into  audio  frequent  currents  to  energize  the  head  telephone. 
It  is  the  function  of  the  oscillation  valve  to  effect  this  conversion. 

14.  ACTION  OF  THE  SIMPLE  VALVE.— In   order   to    make 
clear  the  functioning  of  the  simple  valve  as  a  detector  of  radio  frequency 
currents,  there  is  reproduced  in  Figure  18a,  the  characteristic  curve 
shown  in  Figure  16,  and  in  Figure  18b,  a  circuit  for  demonstrating  ex- 
perimentally the '"valve  action." 

Af  B,  are  the  brushes  of  an  alternating  current  generator.  B-I  is 
a  battery  of  4  volts  to  incandesce  the  filament.  B-2  is  a  second  battery 
to  charge  the  plate  P  to  draw  electrons  from  the  filament.  The  charge 
on  P  is  varied  by  potentiometer  P-2.  As  in  previous  diagrams  the  cur- 

29 


30 


Vacuum  Tubes  In  Wireless  Communication 


rent  of  B-2  is  represented  by  the  single  pointed  arrows,  and  the  electrons 
emitted  by  the  filament  with  double  pointed  arrows,  and  for  this  par- 
ticular illustration  a  single  alternation  of  current  from  generator  A,  B, 
is  shown  by  the  three  pointed  arrows. 


E-l   E-2 

10  ZO  30  40 

E.  M.  F.   OF    B-2 

Figure  18a 


50 


E-4 
60 


E-5 
70 


In  harmony  with  statements  in  paragraph  13  under  (1)  current 
generated  by  A,  B,  can  flow  from  plate  P  to  filament  F,  but  not  in  the 
opposite  direction.  Hence,  from  this  action  by  itself,  an  alternating  cur- 
rent would  be  changed  to  a  direct  current. 

But  there  is  another  important  action  here  to  have  consideration. 


Vacuum  Tubes  In  Wireless  Communication 


31 


Direct  current  from  battery  B-2  flows  constantly  from  plate  P  to  filament 
F,  and  during  the  moment  that  this  current  and  that  of  the  generator 
A,  B,  flow  in  the  same  direction  there  will  be  a  large  increase  of  current 
through  the  telephone  P-l.  But  if  current  from  A,  B,  flows  opposite  to 
that  of  the  battery  there  will  be  a  small  decrease  of  current  through 
the  telephone. 

Just  why  the  telephone  current  increases  to  a  greater  degree  than 
it  decreases  can  be  determined  from  the  characteristic  curve  reproduced 
in  Figure  18a. 

Point  B  on  the  curve  corresponds  to  voltage  E,  E-2,  and  to  current 
a± ;  but  if  the  voltage  is  increased  to  E-3,  the  current  increases  to  a4. 


Figure  18b 

Apparatus  for  demonstrating  the  non-uniform  conductivity  of  the 
vacuum  valve. 


.  From  this  we  see  that  if  the  voltage  of  battery  B-2  is  adjusted  to 
value  E-2  (whatever  it  may  be)  a  small  increase  to  E-3  will  increase  the 
total  current  flowing  through  the  circuit  to  some  value  in  excess  of  »1. 

Conversely,  whe^i  the  E.M.F.  of  the  alternator  opposes  the  E.M.F. 
of  B-2,  the  total  E.  M.  F.  is  less,  hence  the  current  as  read  from  the 
curve  is  slightly  less  than  ax. 

Now,  if  we  substitute  for  the  alternator  a  radio  receiving  trans- 
former, precisely  the  same  action  takes  place,  but  in  this  case  there 
is  impressed  upon  the  valve  a  current  of  extremely  high  frequency.  The 


32 


Vacuum  Tubes  In  Wireless  Communication 


complete  process  by  which  such  radio  frequency  oscillations  are  made 
audible  in  the  head  telephone  can  be  explained  by  the  curves  of  Figure  19. 
The  decaying  groups  of  oscillations  on  the  lower  horizontal  line  0-1 
represent  those  incoming  at  a  given  radio  station  and  the  group  on  the 
line  0-2  represents  the  fluctuations  or  increase  and  decrease  of  the  tele- 


0-3 


AVERAGE  TELEPHONE 
CURRENT 


0-Z 


FLUCTUATING 
TELEPHONE     CURRENT 


INCOMING   OSCILLATIONS 
(MICRO-  AMPS) 


O-l 


Figure  19 

Showing  the  phenomenon  involved  in  the  detection  of  radio  fre- 
quency oscillations  by  the  two-electrode  vacuum  tube.  Graph  O-l  indi- 
cates the  incoming  radio  frequency  oscillations;  graph  O-2,  the  fluctuat- 
ing telephone  current,  and  graph  O-3,  the  average  effect  of  the  telephone 
current  upon  the  diaphragm. 

phone  current.  But  since  the  increase  is  much  greater  than  the  decrease 
it  amounts  in  effect  to  a  uni-directional  current  which  actuates  the  tele- 
phone diaphragm  at  an  audio  frequency.  The  telephone  diaphragm 
responds  to  an  average  effect  of  the  amplitudes  in  0-2,  which,  roughly, 
may  be  represented  by  the  curves  on  line  0-3. 

Beginning  at  the  transmitter,  the  phenomena  involved  may  be 
summed  up  as  follows: 

For  each  spark  at  the  transmitter  the  aerial  radiates  one  group  of 


Vacuum  Tubes  In  Wireless  Communication 


33 


waves  and  each  group  induced  in  the  receiving  apparatus  is  rectified 
by  the  valve  so  that  a  decaying  uni-directional  E.  M.  F.  flows  through 
the  head  telephones,  producing  one  "click"  or  sound.  Therefore,  1,000 
sparks  at  the  sending  station  produce  1,000  clicks  in  the  receiving  tele- 
phone. 

The  condenser  C,  in  the  diagram  of  Figure  18b,  serves  to  store  up 
the  rectified  currents.  It  is  generally  assumed  that  it  discharges  through 
the  head  telephone  aperiodically. 

15.  THE  PRACTICAL  OSCILLATION  VALVE  CIRCUIT  IN 
RADIO. — The  two  batteries  for  the  Fleming  valve  shown  in  Figure  18b, 
are  not  essential  to  its  operation  in  practice.  The  battery  B-l  may  be 
employed  to  render  incandescent  the  filament  and  to  attract  the  electrons 


C-i 


Figure  20 

Circuit  for  the  two-electrode  vacuum  tube  as  an  oscillation  detector 
in  radio  telegraphy.  Current  is  supplied  to  the  plate  circuit  by  the  bat- 
tery B-l,  which  also  incandesces  the  filament. 


from  the  filament  to  the  plate.  The  circuit  is  shown  in  Figure  20.  In 
this  diagram  the  alternating  current  generator  (Figure  18b)  with  brushes 
A,  B,  is  replaced  by  the  primary  and  secondary  circuits  of  a  standard 
radio  receiving  system.  The  aerial  or  antenna  is  shown  at  A,  the  earth 
connection  at  E,  the  primary  winding  of  the  receiving  transformer  at 
L-l,  the  secondary  winding  at  L-2,  and  the  secondary  tuning  condenser 
at  (7-1.  The  telephones  P-2  are  shunted  by  the  condenser  C,  and  the 
battery  B-l  by  the  potentiometer  P-l,  which  is  of  400  ohms  resistance. 
If  the  sliding  contact  of  potentiometer  P-l  is  placed  to  the  right, 
plate  P  will  be  charged  to  a  positive  potential,  the  path  of  the  plate  cur- 
rent being  through  telephones  P-2  through  the  secondary  winding  L-2 
to  plate  P  back  to  the  negative  side  of  filament  F.  But  if  the  contact 
on  P-l  is  placed  to  the  left,  the  potential  of  the  plate  P  in  respect  to  the 
negative  side  of  the  filament  will  be  reduced  to  zero. 


34  Vacuum  Tubes  In  Wireless  Communication 

The  apparatus  shown  in  this  drawing  functions  like  that  in  the 
diagram  of  Figure  18b,  but  in  this  case,  there  is  impressed  across  the 
detector  an  oscillating  E.  M.  F.  occurring  at  a  radio  frequency.  A  half 
cycle  of  the  incoming  oscillations  flows  freely  from  plate  "P  to  filament 
F,  causing  a  large  increase  of  current  through  telephone  P.  The  remain- 
ing half  cycle  flows  oppositely  to  the  plate  current,  causing  a  small 
decrease  in  the  telephone  current.  The  net  result  is  an  increase  of  cur- 
rent in  one  direction  through  the  head  telephone. 

To  comprehend  this  clearly,  the  student  must  keep  in  mind  the 
characteristic  curve  shown  in  Figure  18a,  and  note  the  deductions  which 
may  be  drawn  therefrom.  When  the  operator  adjusts  the  potentiometer 
P-l  for  the  maximum  strength  of  signals,  he  operates  the  valve  on  some 
point  of  the  characteristic  curve  where  the  addition  of  a  slight  antenna 
E.  M.  F.  produces  either  a  large  increase  or  a  large  decrease  of  current 
through  the  head  telephones. 

Thus  at  point  C  (Figures  16  or  18a)  the  effect  of  a  group  of  radio 
frequency  oscillations  is  evidently  to  cause  a  decrease  in  telephone  cur- 
rent, but  at  point  B  the  reverse  is  the  case. 

Careful  study  of  Figure  18 a,  reveals  the  points  on  the  curve  to  which 
the  plate  current  should  be  adjusted  for  maximum  signals  under  different 
values  of  oscillatory  E.  M.  F.  supplied  by  the  antenna.  If  the  incoming 
signals  are  relatively  strong,  the  plate  voltage  may  be  adjusted  to  a 
point  slightly  below  the  bend  of  the  curve  B.  This  increases  the  tele- 
phone current  to  a  greater  value  than  could  be  obtained  further  along 
the  curve.  But  if  the  incoming  signals  are  relatively  weak,  the  plate 
potential  must  be  adjusted  to  a  point  further  along  the  bend  in  order 
that  the  E.M.F.  of  the  incoming  signal  may  change  the  plate  current 
sufficiently  so  that  the  telephone  current  will  increase  to  a  greater  ex- 
tent than  it  decreases.  The  foregoing  applies  equally  well  to  the  upper 
bend  of  the  curve. 

In  the  parlance  of  practice,  the  plate  voltage  that  will  give  the 
maximum  strength  of  signals  for  strong  incoming  oscillations  generally 
is  not  the  voltage  that  will  give  the  best  response  to  weak  signals. 

It  should  be  mentioned  here,  that  the  resistance  of  the  valve  from 
plate  to  filament  is  very  high.  Hence,  the  higher  the  potential  produced 
by  a  given  group  of  incoming  oscillations  the  stronger  the  response 
obtained.  Therefore,  the  secondary  winding  of  the  receiving  tuner 
should  be  designed  to  give  the  maximum  possible  voltage  for  a  given 
group  of  incoming  oscillations.  This  requirement  is  met  by  a  secondary 
inductance  which  requires  a  very  small  shunt  condenser  for  resonance 
with  the  incoming  signal.  In  general,  the  capacity  of  (7-1,  Figure  20, 


Vacuum  Tubes  In  Wireless  Communication 


35 


should  not  exceed  .0001  microfarads  when  tuning  for  the  ordinary  com- 
mercial wave  length. 

The  valve  of  Figure  20  may  be  adjusted  to  the  incoming  radio  sig- 
nals as  follows:  The  temperature  of  the  filament  F  may  be  definitely 
fixed  by  means  of  a  small  ammeter,  followed  by  adjustment  of  the  po- 
tentiometer P-l  until  the  maximum  strength  of  signals  is  obtained.  In 
event  that  an  ammeter  is  not  provided,  the  operator  must  try  various 
degrees  of  incandescence  with  simultaneous  adjustment  of  the  poten- 
tiometer P-l  until  maximum  signals  are  obtained  in  the  receiving  tele- 
phone. 

(a)  Fleming  Valve  Circuits  in  Simple  Form.  The  Fleming  valve 
may  be  employed  as  an  oscillation  detector  without  the  use  of  an  aux- 
iliary E.  M.  F.  Very  good  results  are  often  so  obtained.  The  circuit 
is  shown  in  Figure  20a.  Filament  F  is  incandesced  by  battery  B-l.  One 


L-i,    ,.L-2 


C-l 


C-2 


P-l 


Figure  20a 

Simple  circuit  for  the  two-electrode  vacuum  tube 
as  an  oscillation  detector  in  radio  telegraphy. 


terminal  of  the  secondary  winding  L-2  is  attached  to  the  plate  P  and  the 
other  to  the  negative  side  of  the  filament  F.  The  action  of  the  valve  is 
as  follows :  When  an  oscillating  E.  M.  F.  is  impressed  upon  the  plate  and 
filament,  the  plate  P  is  charged  alternately  to  a  positive  and  negative 
potential.  When  plate  P  is  charged  to  a  positive  potential  electrons  are 
drawn  over  from  the  filament,  which  is  equivalent  to  saying  that  a  semi- 
cycle  flows  from  P  to  F.  When  P  is  charged  negatively,  no  electrons 
are  drawn  over  to  the  plate  and  consequently  no  current  passes  the  valve. 
The  condenser  0-2,  therefore,  receives  a  uni-directional  charge  over  the 
duration  of  a  wave  train  and  at  the  termination  thereof  discharges 
through  telephone  P-l.  For  best  results  the  telephone  should  have  a  re- 
sistance of  several  thousand  ohms. 


36 


Vacuum  Tubes  In  Wireless  Communication 


(b)  Other  Circuits  for  the  Vacuum  Valve.  In  the  circuit  shown 
in  Figure  20b,  two  batteries  are  employed  with  the  two-electrode  valve. 
The  battery  5-1,  as  usual,  renders  incandescent  the  filament,  and  the 
battery  B-2  supplies  the  local  telephone  current.  It  is  to  be  noted  that 
the  terminals  of  the  telephone  circuit  are  attached  to  the  plate  P,  and 
the  negative  side  of  the  filament  F.  In  practice,  the  operator  carefully 
regulates  the  incandescence  of  the  filament  F,  and  adjusts  the  potential 
of  the  battery  B-2  until  maximum  response  is  secured  in  the  head  tele- 
phones. 


7F..C-1 


Figure  20b 
Modified  circuit  for  the  Fleming  valve. 


_      P-i 
Figure  20c 
Unipolar  connection  for  the  two-electrode  valve. 

The  circuit  shown  in  Figure  20c  is  somewhat  similar  to  Figure  2 Ob, 
with  the  exception  that  the  negative  side  of  the  filament  F  is  connected 
to  one  terminal  of  the  secondary  circuit.  Additional  uni-polar  connec- 
tions for  the  vacuum  tube  will  be  shown  in  Part  VI.  The  adjustment 
of  the  apparatus  in  Figure  20c  is  similar  to  that  in  Figure  20b.  The 
open  and  closed  circuits  of  the  receiving  tuner  are  carefully  tuned  to 
the  incoming  signal,  and  the  incandescence  of  the  filament  F  and  the 


Vacuum  Tubes  In  Wireless  Communication 


37 


potential  of  the  battery  B-2  carefully  regulated  until  maximum  response 
is  secured. 

The  Fleming  valve  connected  as  in  Figures  20  to  20c,  constitutes 
a  sensitive  and  very  stable  oscillation  detector  in  radio  telegraphy  and 
has  been  much  used  in  ship  and  shore  stations. 


TO  BATTERY 


Figure  21 

The  fundamental  construction  of  the  three-elec- 
trode vacuum  tube  often  called  the  electron  relay. 

16.  THE  THREE-ELECTRODE  VALVE.— A  wider  range  of 
circuits,  and  a  greater  ease  of  control  of  the  vacuum  valve  as  a  detector 
in  radio  is  secured  by  interposing  what  is  termed  a  grid  element  between 
the  filament  and  plate.  The  grid  element  valve  as  a  detector  in  wireless 
telegraphy  is  credited  to  Dr.  Lee.DeForest.  A  valve  so  constructed  is 
shown  in  Figure  21.  F  is  usually  a  tantulum  or  tungsten  filament;  Of 
the  grid  of  tungsten,  nickel  or  copper;  and  P,  a  nickel  or  aluminum  plate. 
The  grid,  filament,  and  plate  may  assume  different  shapes  than  herein 
shown,  and  may  differ  in  material  and  construction,  but  the  metals  just 
mentioned  are  used  in  some  types  of  valves.  In  one  well-known  type  of 
valve,  the  filament,  grid,  and  plate  are  made  of  tungsten. 

The  immediate  effect  of  inserting  the  grid  element  can  only  be  under- 
stood by  clearly  comprehending  the  conditions  which  would  exist  in  its 
absence.  Assuming  the  grid  to  be  removed,  the  filament  heated,  and  the 
plate  battery  connected,  it  is  clear  that  an  electrostatic  field  is  maintained 
between  the  plate  and  filament,  and  that  every  point  in  this  space  will 
have  some  potential  between  that  of  the  plate  and  that  of  the  filament. 
It  is  this  electrostatic  field  which  causes  the  passage  of  electrons  from 
the  filament  to  the  plate. 


38 


Vacuum  Tubes  In  Wireless  Communication 


When  the  grid  is  inserted  into  the  space  between  the  plate  and  fila- 
ment, we  should  naturally  expect  the  grid  to  have  the  potential  which  the 
electrostatic  field  has  at  this  point.  This,  however,  is  not  the  case,  for 
the  grid  captures  electrons  and  thus  has  its  potential  lowered.  It  may 
therefore  finally  become  either  positive  or  negative  to  the  filament,  but 
it  will  always  be  negative  as  compared  with  the  potential  which  the 
space  it  occupies  would  have  if  it  were  removed.  This  has  been  experi- 
mentally proven  by  Armstrong. 


1.0 


G-l 


-IO  -5  0  +5  +  10 

GRID    POTENTIAL 

Figure  22 

Grid — potential   plate — current   characteristic   curve   of  the   three- 
electrode  vacuum  tube. 


This  lowered  potential  of  the  grid  0&s£n/c£s  the  flow  of  electrons  so 
that  less  escape  to  the  plate.  As  in  the  two-electrode  valve  (in  order 
to  withdraw  electrons  from  the  filament),  the  positive  pole  of  a  battery 
B-2  is  connected  to  the  plate,  but  if  the  grid  element  is  charged  suffi- 
ciently negatively  by  an  external  E.M.F.,  the  electron  current  will  be 
completely  cut  off.  Normal  flow  of  current  (from  plate  to  filament)  can 
only  be  secured  by  charging  the  grid  to  a  lower  negative  or  to  a  positive 
potential.  The  student  may  now  obtain  a  hint  how  the  variation  of  the 
grid  potential  affects  the  strength  of  current  in  the  plate  circuit.  For 
example,  the  amount  of  current  flowing  from  P  to  F  may  be  varied  by 
simply  changing  the  potential  of  the  grid  element  G.  That  is,  the  grid 
may  be  employed  to  open  and  close  the  plate  circuit  or  to  change  its 
resistance. 

Now,  as  already  mentioned,  the  potential  of  the  grid  may  be  sev- 
eral volts  negative  to  the  negative  side  of  the  filament,  or  it  may  be  the 


Vacuum  Tubes  In  Wireless  Communication  39 

same  as  the  negative  side  of  the  filament,  or  it  may  be  positive  with 
respect  to  the  field  in  the  plane  of  the  grid  which  would  exist  if  the  grid 
were  removed  from  the  bulb  ;*  but  it  will  always  be  negative  with  respect 
to  the  potential  of  the  field  in  the  plane  of  the  grid  which  would  exist 
if  the  grid  were  removed  from  the  bulb. 

Just  how  variation  of  the  grid  potential  by  an  external  E.M.F. 
effects  the  flow  of  the  plate  current  will  be  readily  understood  by  the 
plot  of  a  characteristic  curve  of  the  three-electrode  tube.  We  have 
already  shown,  in  Figure  16,  a  characteristic  curve  of  the  two-electrode 
valve. 

(a)  Characteristic  Curve  of  Three-Electrode  Valve.  Since  there 
are  three  elements  in  this  form  of  valve  it  is  evident  that  several  different 
groups  of  characteristic  curves  may  be  taken.  Thus  there  is  a  plate- 
voltage  to  plate-current  group  of  curves  dependent  on  temperature  of  the 
filament,  and  a  grid- voltage  to  grid-current  group  dependent  on  the 
temperature  of  the  filament,  and  a  grid-voltage  to  plate-current  group 
dependent  upon  the  filament  temperature  and  plate  voltage.  These  are 
all  related  in  important  but  complex  ways  and,  as  in  the  case  of  the  two- 
element  bulb,  the  operation  is  by  no  means  completely  understood.  We 
shall  consider  here  only  the  grid  voltage-plate  current  curve  upon  which 
the  most  commonly  accepted  explanation  of  the  tubes'  operation  are 
usually  based. 

In  the  curve  for  the  three-electrode  valve  shown  in  Figure  22,  the 
horizontal  axis  indicates  the  potential  of  the  grid  in  respect  to  the  fila- 
ment, and  the  vertical  axis,  the  plate  current  in  milliamperes.  The  differ- 
ence between  the  curve  of  Figure  22  and  that  of  Figure  16,  is  that  the 
latter  shows  the  volt-ampere  relation  in  the  plate  circuit  of  the  two- 
electrode  valve,  whereas  Figure  22  gives  the  values  of  the  plate  current 
under  different  values  of  positive  and  negative  grid  potential. 

The  connections  for  the  apparatus  by  which  the  data  for  this  curve 
were  obtained  appear  in  Figure  23.  Here  P-l  is  a  potentiometer ;  B-2, 
the  batteiy  for  the  plate  circuit,  45  to  200  volts,  and  A,  a  milliammeter. 
The  filament  current  is  regulated  by  the  10-ohm  rheostat  R.  The  grid 
may  be  charged  either  positively  or  negatively  by  sliding  the  potentio- 
meter contact  P-3  towards  B  or  A. 

The  data  for  Figure  22,  was  secured  by  holding  the  voltage  of  the 
battery  B-2  at  some  constant  value  while  changing  the  potential  of  the 


*See  Proceedings  of  the  Institute  of  Radio  Engineers,  Sept.,  1915,  Vol.  3, 
No,  3, 


40 


Vacuum  Tubes  In  Wireless  Communication 


grid  to  a  positive  or  negative  value.  It  will  be  observed  that  the  plate 
current  with  this  particular  valve  is  maximum  when  the  grid  is  charged 
to  about  three  volts  positive  potential.  With  the  grid  at  approximately 
five  volts  negative  potential,  the  current  in  the  plate  circuit  is  nearly 
zero.  On  the  straight  part  of  the  curve  A1  to  .B1  the  plate  current  is 
directly  proportional  to  the  grid  potential.  Hence,  if  an  alternating 
E.M.F.  of  not  too  great  value  is  impressed  upon  the  grid  and  filament, 
the  plate  current  will  rise  and  fall  uniformly. 

The  curve  of  Figure  22  will  repay  careful  study,  for  it  is  upon  the 
operating  characteristics  therein  shown,  that  the  at  present  accepted 
explanations  of  the  fundamental  actions  of  the  three-electrode  vacuum 
valve  as  a  detector  and  amplifier  in  radio-telegraphy  are  based. 


A'    " 


Figure  23 

Connections  of  the  apparatus  for  obtaining  the 
characteristic  curve  of  the  three-electrode  tube. 


By  way  of  illustration  it  is  clear,  from  Figure  22,  that  if  the  grid 
has  been  adjusted  to  a  fairly  high  negative  potential,  for  example,  that 
indicated  by  point  A,  a  few  volts  decrease  (of  the  grid  potential)  permits 
a  large  mcrease  of  current  in  the  plate  circuit,  as  may  be  seen  by  fol- 
lowing the  dotted  line  from  A1  to  the  vertical  axis.  On  the  other  hand,  if 
the  grid  potential  is  increased  negatively  by  the  same  amount,  the  plate 
current  decreases  by  a  relatively  smaller  amount.  Hence,  if  the  valve  is 
employed  as  a  detector  of  radio  frequency  oscillations  and  the  grid  poten- 
tial is  adjusted  to  point  A  on  the  curve,  Figure  22,  the  rise  and  fall 
of  the  grid  potential  occasioned  by  the  incoming  oscillations  produces 
a  current  of  similar  frequency  in  the  plate  circuit,  the  amplitudes  of  the 
increase  exceeding  those  of  the  decrease.  Hence,  what  amounts,  in  so  far 
as  the  telephone  is  concerned,  to  a  uni-directional  current  charges  the 
telephone  condenser  which  discharges  through  the  telephone  once  for 
each  group  of  incoming  oscillations.  It  is  also  clear  that  if  the  valve  is 
worked  at  the  upper  bend  of  the  curve,  Figure  22,  the  foregoing  actions 


Vacuum  Tubes  In  Wireless  Communication  41 

will  be  reversed,  i.e.,  a  group  of  incoming  oscillations  will  cause  the  tele- 
phone current  to  decrease  rather  than  increase. 

The  curve  will  be  discussed  again  in  paragraph  17. 

(b)  Valve  Terminology.  In  order  to  prevent  confusion  from  this 
point  on,  we  shall  refer  to  the  circuit  containing  the  plate,  the  local 
battery,  and  the  head  telephone  as  the  "plate  circuit"  and  to  the  second- 
ary circuit  including  the  secondary  tuning  coils,  condensers,  potentio- 
meter, battery,  and  grid  as  the  "grid  circuit." 

Thus,  in  Figure  24,  the  grid  circuit  includes  L-2,  C-l,  P-l,  £-3,  0-2, 
G,  to  the  negative  side  of  F.  The  plate  circuit  embraces  P,  negative  side 
of  F,  B-2,  P-2,  and  the  vacuous  space  from  P  to  F.  It  is  also  customary  to 
call  the  grid  circuit  the  input  circuit,  and  the  plate  circuit  the  output 
circuit.* 

We  shall  frequently  refer  to  the  current  of  battery  B-2  as  the  "plate 
current"  and  the  necessary  changes  of  grid  potential  will  be  effected  by 
the  "grid  lattery"  B-3. 

We  shall  also  speak  of  the  audio  frequency  and  radio  frequency 
"component"  of  the  plate  current.  These  terms  will  be  explained  in 
detail  further  on. 

17.  THE  THREE-ELECTRODE  VALVE  AS  AN  ELECTRON 
RELAY  AND  RECTIFIER.— 

(a)  The  Valve  as  an  Electron  Relay  (without  Grid  Condenser). 
As  already  mentioned,  the  only  fundamental  operating  characteristic 
of  the  vacuum  valve  as  an  oscillation  detector  of  which  we  shall  make 
use  is  the  graph  of  Figure  22.  We  shall  show  more  in  detail  that  the 
three-electrode  valve  may  be  employed  as  a  relay  or  as  a  combined  recti- 
fier and  amplifier. 

A  complete  circuit  for  taking  advantage  of  the  non-uniform  char- 
acteristic of  the  three-element  tube  is  shown  in  Figure  24,  where  the 
plate  P  as  usual  is  charged  positively  by  the  battery  B-2  in  series  with 
which  is  the  telephone  P-2.  Filament  F  is  incandesced  by  battery  J5-1 
and  the  strength  of  the  current  regulated  by  rheostat  R. 

The  potential  of  the  grid  to  filament  is  definitely  fixed  by  battery 
J5-3  shunted  by  potentiometer  P-l.  Condenser  0-2  provides  a  path  for 
the  radio  frequency  currents  about  the  potentiometer  P-l.  The  primary 
winding  of  the  usual  receiving  tuner  is  indicated  at  L-I  and  the  second- 
ary winding  at  L-2. 


"The  filament  circuit  including  the  lighting  battery  and  the  rheostat  is  often 
termed  the  "A"  circuit,  and  the  plate  circuit  the  "B"  circuit, 


42 


Vacuum  Tubes  In  Wireless  Communication 


Jj 


,( 0000000 


Vacuum  Tubes  In  Wireless  Communication  43 

(b)  Functioning  of  the  Valve  as  Explained  from  the  Character- 
istic Curve.  The  manner  in  which  the  valve  functions  in  Figure  24 
has  heen  partially  explained  in  a  preceding  paragraph.  Reference 
should  again  be  made  to  Figure  22.  Assume,  for  example,  the  potential 
of  the  grid  in  respect  to  the  filament  is  that  corresponding  to  point  B, 
that  is  the  grid  and  filament  have  the  same  potential;  then  a  negative 
charge  applied  to  the  grid  produces  a  decrease  in  the  plate  current 
and  a  positive  charge  imparted  to  the  grid  produces  an  increase  in  the 
plate  current. 

Hence,  if  an  alternating  E.M.F.  such  as  a  group  of  incoming  radio 
frequency  oscillations  is  impressed  upon  the  filament  and  the  grid,  the 
plate  current  will  rise  and  fall  at  the  frequency  of  the  impressed 
E.M.F.,  i.e.j  the  incoming  radio  frequency  current  will  be  repeated  in 
the  plate  circuit,  but  often,  with  increased  amplitude  owing  to  the 
radio  frequency  amplifying  action  of  the  valve. 

This  current,  however,  will  not  be  heard  in  the  receiving  telephone 
because  its  frequency  is  above  audition  and  the  positive  and  negative 
halves  are  of  equal  amplitude.  But,  as  the  curve  of  Figure  22  clearly 
indicates,  if  the  potential  of  the  grid  is  adjusted  by  a  potentiometer 
to  point  A,  a  small  decrease  in  the  grid  potential,  (i.e.,  a  change  toward 
zero)  causes  a  comparatively  large  increase  of  the  plate  current;  on 
the  other  hand,  an  increase  of  the  grid  potential  (in  a  negative  sense) 
causes  a  relatively  small  decrease  of  the  plate  current.  Hence,  if  the 
grid  and  filament  are  connected  to  the  secondary  terminals  of  a  receiv- 
ing tuner,  what  amounts  to  a  rectified  current  will  flow  in  the  plate  or 
output  circuit,  that  is,  the  average  increase  in  current  occasioned  by  the 
positive  halves  of  the  incoming  oscillations  exceeds  the  average  decrease 
in  current  due  to  the  negative  halves  of  the  incoming  oscillations.  This 
current  charges  the  telephone  condenser  0-3,  Figure  24,  which  dis- 
charges through  the  telephone  probably  in  one  direction.  As  already 
explained  in  connection  with  the  two-electrode  tube,  if  the  grid  potential 
is  adjusted  to  the  upper  bend  of  the  curve,  the  foregoing  action  is 
reversed.  The  effect  of  a  group  of  incoming  oscillations  then  is  to 
decrease  the  plate  current. 

Since  each  group  of  incoming  oscillations  causes  a  large  increase 
of  current  through  the  head  of  the  telephone,  the  diaphragm  will 
produce  one  click  for  each  spark  discharge  at  the  transmitter. 

It  is  apparent  from  the  characteristic  curve  that  the  repeated  plate 
current  may  have  uniform  increase  or  decrease,  or  it  may  assume  the 
nature  of  a  rectified  current  depending  upon  the  point  on  the  char- 
acteristic curve  at  which  the  valve  is  worked.  Thus,  near  the  upper 
bend  C,  and  the  lower  bend  A,  a  rectified  current  will  flow  in  the  plate 


44 


Vacuum  Tubes  In  Wireless  Communication 


circuit,  but  along  the  straight  portion  of  the  curve  B,  equal  increase 
and  decrease  of  the  plate  current  will  be  obtained.  In  the  latter  con- 
dition, the  valve  is  properly  adjusted  for  amplification  by  a  second 
tube,  i.e.,  cascade  amplification  at  radio  frequencies. 

In  the  diagram  of  Figure  24,  the  grid  battery  is  connected  in 
the  grid  circuit  to  take  advantage  of  the  non-uniform  properties  of  the 


o-i 


INCOMING 
OSCILLATIONS 


GRID  TO  FILAMENT 
POTENTIAL 


0-2 


0-3 


RADIO  FREQUENCY 
COMPONENT-  PLATE 
CURRENT 


0-4 


TELEPHONE    CURRENT 


Figure  25 

Curves  showing  how  the  vacuum  valve  used  as  an  assymetrical 
relay  produces  audible  response  in  the  receiving  telephone. 


tube,  but  it  should  be  understood  that  the  tube  will  function  in  the 
same  way  without  the  grid  battery;  that  is,  if  a  good  sample  of  a 
valve  is  used,  by  careful  adjustment  of  the  filament  temperature  and 


Vacuum  Tubes  In  Wireless  Communication 


45 


the  plate  current,  the  operator  can  obtain  the  best  operating  char- 
acteristic for  a  given  condition  of  service.  In  other  words,  he  thus 
locates  the  point  on  the  curve  which  will  give  the  loudest  response  in 
the  head  telephone  without  employing  a  grid  battery. 

(c)  Relaying  Action.  We  have  shown  that,  owing  to  the  relaying 
action  of  the  valve,  a  current  of  any  wave  form  impressed  upon  the 
grid  circuit  may  be  repeated  with  amplification  in  the  plate  circuit. 
This  relaying  effect  is  obtained  by  reason  of  the  phenomenon  shown 
by  the  characteristic  curve,  i.e.,  a  fractional  amount  of  energy  impressed 
upon  the  grid  circuit  causes  a  larger  variation  of  current  in  the  plate 
circuit.  Owing  to  the  extreme  mobility  of  electrons  under  the  influence 


p-i 


Figure  26 

The  circuit  for  the  use  of  the  valve  as  a  combined  rectifier  and 
amplifier.  Incoming  oscillations  are  rectified  and  stored  up  in  the 
grid  condenser,  the  charge  and  discharge  of  which  varies  the  telephone 
current  at  an  audio  frequency. 

of  positive  and  negative  electrostatic  fields,  this  relaying  action  may 
be  secured  both  at  audio  and  at  radio  frequencies. 

The  result  is  analogous  to  that  of  an  ordinary  land  line  telegraph 
relay  in  which  feeble  line  currents  energize  a  magnet  winding  of  a 
relatively  great  number  of  ampere  turns.  In  front  of  the  magnet 
poles  is  mounted  a  light  armature  carrying  a  platinum  point  which,  in 
turn,  makes  contact  with  a  stationary  contact  closing  the  circuit  of  a 
more  powerful  battery  including  in  its  circuit  a  telegraph  sounder; 
greater  volume  of  sound  is  secured  from  the  sounder  because  of  the 
greater  strength  of  the  local  battery  current  compared  to  the  current 
which  flows  through  the  windings  of  the  relay. 


46  Vacuum  Tubes  In  Wireless  Communication 

Diagrammatically,  the  actions  of  the  valve  as  a  relay  for  reception 
of  signals  in  wireless  telegraphy  is  shown  in  Figure  25.  0-1  represents 
the  incoming  oscillations ;  0-2,  the  potential  of  the  grid  to  filament ;  0-3, 
the  radio  frequency  fluctuations  of  the  plate  current,  and  0-4,  the  ap- 
proximate average  telephone  current.  It  is  readily  seen  in  graph  0-4 
that  the  effect  of  a  single  group  of  incoming  oscillations  is  to  increase 
the  telephone  current  at  an  audio  frequency.  This  is  the  action  in  the 
region  of  the  lower  bend  of  the  curve.  At  the  upper  bend  the  action 
is  reversed. 

If  the  grid  is  maintained  at  a  negative  potential  by  the  grid  bat- 
tery, it  prevents  the  flow  of  a  conduction  current  between  the  grid  and 
filament.  Hence,  the  grid  circuit  absorbs  no  energy  from  the  incoming 
oscillations.  This  reduces  the  damping  of  the  secondary  circuit. 

(d)  Use  of  the  Three-Electrode  Valve  with  Grid  Condenser.  The 
circuit  shown  in  Figure  26,  differs  from  that  of  Figure  22,  in  that  it 
employs  the  so-called  grid  condenser,  C,  connected  between  the  grid 
and  the  upper  terminal  of  the  secondary  coil  of  the  receiving  tuner. 
The  function  of  the  grid  condenser  is  to  store  up  the  currents  which 
are  rectified  by  the  valve  action  between  the  grid  and  filament.  As 
will  be  explained  further  on,  the  charge  and  discharge  of  this  con- 
denser during  the  reception  of  damped  oscillations  in  radio  telegraphy 
decreases  and  increases  the  plate  current  at  an  audio  frequency. 

The  action  is  somewhat  as  follows:  When  the  incoming-  oscilla- 
tions tend  to  charge  the  grid  negatively  no  current  flows  from  the  grid 
to  the  filament,  but  when  the  grid  is  charged  positively  current  passes 
from  the  grid  to  filament,  and  the  grid  condenser  therefore  receives  a 
uni-directional  charge  over  the  duration  of  a  wave  train.  Therefore 
a  charge  of  increasing  strength  piles  up  in  the  grid  condenser  C,  which 
is  negative  on  the  grid  side.  This,  as  is  clear  from  the  fundamental 
curve  of  Figure  22,  obstructs  more  and  more  the  passage  of  electrons 
from  the  filament  to  the  plate,  causing  a  decrease  in  the  plate  current. 
At  the  termination  of  a  group  of  incoming  oscillations,  the  charge  in 
the  grid  condenser  leaks  off  either  through  the  valve  itself,  or  through 
a  special  leak  resistance  of  several  thousand  ohms  shunting  the  grid 
condenser.  The  grid  then  returns  to  normal  potential  and  likewise  the 
plate  current.  It  is  evident  that  each  spark  at  the  transmitter  eventually 
reduces  the  telephone  current  at  the  receiver.  In  other  words,  the 
telephone  current  varies  as  the  spark  frequency  of  the  transmitter. 

During  the  time  that  the  incoming  oscillations  undergo  rectification, 
the  potential  of  the  grid  fluctuates  at  a  radio  frequency  and,  just  as  in 
the  case  of  Figure  24,  the  plate  current  rises  and  falls  at  a  radio  fre- 


Vacuum  Tubes  In  Wireless  Communication 


47 


quency,  but  this  current  is  not  heard  in  the  head  telephone.  Although 
this  repeated  radio  frequency  current  is  not  heard  in  the  head  tele- 
phone it  is  put  to  account  in  the  regenerative  and  amplification  circuits 
described  further  on. 

Reviewing  the  foregoing,  it  is  seen  that  two  results  are  obtained 
simultaneously  in  the  plate  circuit.  During  the  time  that  the  rectified 
oscillations*  are  building  up  a  charge  in  the  grid  condenser,  the  cur- 
rent in  the  plate  circuit  decreases,  but  when  the  charge  leaks  out  the 


10- 1 


INCOMING 
OSCILLATIONS 


0-2  — 


0-5 


VARIATION  OF 
PLATE.  CURRLNT 


0-4 


TELEPHONE  CURRENT 

A 


Figure  27 

Graphs  showing  the  operating  phenomena   of  the  three-electrode 
tube  as  an  oscillation  detector,  with  a  series  or  grid  condenser. 


*  Rectification  occurs  in  the  vacuum  tube  in  the  following  way :  the  elec- 
tronic emission  of  the  filament  is  in  the  direction  of  the  grid,  and  in  terms 
of  the  electronic  theory,  negative  electricity  can  pass  from  the  filament  to  the 
grid  but  not  in  the  opposite  direction,  which  is  the  same  as  saying  in  terms  of 
the  old  theory,  that  current  can  pass  from  the  grid  to  the  filament  but  not  in 
the  opposite  direction.  When  a  group  of  radio  frequency  oscillations  are 
impressed  upon  the  grid  circuit  each  succeeding  half  of  the  incoming  wave  train 
tends  to  increase  the  charge  in  the  grid  condenser.  The  wave  train  is  thus 
rectified. 


48  Vacuum  Tubes  In  Wireless  Communication 

condenser,  the  plate  current  returns  to  normal  value.  This  reduction 
follows  the  spark  at  the  transmitter.  Meanwhile  the  incoming  oscilla- 
tions are  repeated  in  the  telephone  circuit. 

Diagrammatically,  the  actions  of  a  three-electrode  tube  with  a  grid 
condenser  can  be  shown  as  in  Figure  27,  where  graph  0-1  shows  two 
groups  of  incoming  oscillations;  graph  0-2,  the  potential  of  the  grid  in 
respect  to  the  filament;  graph  0-3,  the  corresponding  reduction  of  the 
plate  current  and  the  superposed  or  repeated  radio  frequency  current; 
and  graph  0-4,  the  approximate  average  telephone  current. 

(e)  Radio-  and  Audio-Frequency  Component  of  the  Plate  Cur- 
rent. The  student  should  give  attention  to  the  curve  0-3  of  Figure  27. 
Here    the    successive    variations    of    the    plate    current    in    0-3    are 
those  of  the  repeated  radio  frequency  current  and  they  are  termed  the 
radio  frequency  component  of  the  plate  current.     The  large  depression 
A  in  the  oscillogram  0-4,  represents  the  reduction  of  the  plate  current 
occasioned  by  the  charge  which  accumulates  in  the  grid  condenser  dur- 
ing a  group  of  incoming  oscillations.     This  variation  is  termed  the 
audio  frequency  component  of  the  plate  current. 

In  order  to  prevent  an  extraordinary  potential  from  piling  up  upon 
the  condenser  C,  a  battery  and  potentiometer  are  often  connected  around 
the  grid  condenser  to  control  definitely  the  charge  accumulating  in  it. 
For  instance,  if  the  grid  became  charged  to  an  excessive  negative  poten- 
tial, the  plate  current  would  be  reduced  to  zero  and  the  valve  rendered 
inoperative. 

The  student  will  now  observe  that  the  apparatus  in  the  diagrams, 
Figures  24  and  26,  functions  in  a  manner  similar  to  the  diagram  of 
Figure  20. 

With  the  connections  of  the  diagram  of  Figure  24,  it  is  desirable 
that  the  potential  of  the  grid  be  carefully  regulated  by  the  grid  battery 
in  order  that  best  advantage  may  be  taken  of  the  non-uniform  con- 
ducting properties  of  the  valve,  for  at  the  points  on  the  characteristic 
curves  where  the  plate  current  increases  and  decreases  by  unequal 
amounts,  the  maximum  sound  will  be  obtained  from  the  head  telephone. 

(f)  Summary  of  the  Phenomena  of  the  Characteristic  Curve.   In 
addition  to  the  deductions  which  may  be  drawn  from  the  characteristic 
curve  of  Figure  22,  some  of  which  have  been  mentioned  in  preceding 
paragraphs  and  here  repeated,  additional  phenomena  are  presented. 

As  already  stated,  if  the  grid  potential  corresponds  to  that  of  point 
A  on  the  lower  bend  of  the  curve,  the  amplitude  of  the  positive  halves 
of  the  plate  current  occasioned  by  the  incoming  oscillations  exceeds  the 


Vacuum  Tubes  In  Wireless  Communication 


49 


amplitude  of  the  negative  halves.  But  if,  on  the  other  hand,  the  grid 
potential  is  adjusted  to  the  upper  bend  or  point  (7,  the  former  condition 
will  be  reversed,  that  is,  the  amplitude  of  the  negative  halves  of  the 
repeated  radio  frequency  oscillations  in  the  plate  circuit  will  exceed 
the  amplitude  of  positive  halves  which  is  just  opposite  to  that  in  the  pre- 
ceding case.  At  any  point  along  the  straight  slope  of  the  curve,  such  as 
from  A1  to  B1,  approximately  equal  increases  and  decreases  of  the  plate 
current  will  be  obtained  provided  the  alternating  E.M.F.  impressed  upon 
the  grid  is  not  too  great. 

Again,  if  the  valve  is  operated  at  point  A  and  the  voltage  im- 
pressed upon  the  grid  by  an  oscillating  E.M.F.  of  radio  frequency  is 
but  a  small  fraction  of  a  volt,  substantially  equal  increases  and  decreases 
of  the  plate  current  will  be  secured,  but  no  response  will  be  obtained 
in  the  head  telephone  for  as  already  explained,  the  telephone  will  not 
respond  to  radio  frequencies. 

If  the  valve  is  to  be  used  as  an  amplifier,  as  will  be  described 
further  on,  and  it  is  desired  that  the  plate  current  increase  and  decrease 


I!  fix 


,, 


,-C-z 

* 


zSIS™1* 


Figure  28 

The  three-electrode  vacuum  valve  as  an  oscillation  detector  with 
a  tuned  plate  circuit. 


by  equal  amounts,  the  valve  may  be  worked  at  the  upper  or  lower 
bends  (at  point  A  or  point  (7),  for  small  impressed  E.M.F. 's.  But,  as 
stated  before,  if  the  voltage  impressed  upon  the  grid  is  rather  high, 
then  a  distortionless  current  will  be  obtained  in  the  plate  circuit  for 
any  given  E.M.F.  impressed  upon  the  grid  circuit,  only  along  the  slope 
of  the  curve  A1  to  B1.  This  however  is  not  true  in  all  cases. 

18.  THE  TUNED  PLATE  CIRCUIT.— Further  amplification  of 
incoming  radio  signals  can  be  obtained  by  tuning  the  plate  circuit  of  the 
vacuum  valve.  Armstrong's  method  is  shown  in  Figure  28,  but  since 


50  Vacuum  Tubes  In  Wireless  Communication 

this  is  essentially  a  regenerative  system,  it  will  be  considered  more  in 
detail  in  connection  with  other  tuned  plate  circuits  in  Part  IV. 

In  addition  to  the  apparatus  of  previous  diagrams,  this  circuit 
includes  a  plate  circuit  inductance  L-3  and  a  condenser,  C-3.  The  coil 
L-3,  in  conjunction  with  the  electrostatic  capacity  of  the  valve  (between 
F  and  P)  constitutes  an  oscillation  circuit  of  variable  frequency.  The 
condenser  C-2  acts  as  a  by-pass  for  the  radio  frequency  component  of 
the  plate  current  around  the  head  telephone  P-l  and  the  battery  B-2 
Condenser  C-2  may  be  of  fixed  capacity. 

In  brief,  as  the  incoming  radio  frequency  oscillations  are  repeated 
into  the  plate  circuit,  the  counter  E.M.F  of  coil  L-3  either  assists  or 
opposes  the  plate  circuit  battery  according  to  whether  the  latter  de- 
creases or  increases.  The  effect  of  this  counter  E.M.F.  is  to  alternately 
decrease  and  increase  the  potential  difference  between  F  and  P,  and 
since  the  space  between  F  and  P  constitutes  a  condenser  of  small  capacity, 
the  charge  accumulated  therein  is  varied  in  accordance. 

As  will  be  explained  in  Part  IV,  the  internal  or  self-capacity  of  the 
valve  acts  as  a  coupling  to  transfer  energy  from  the  plate  circuit 
to  the  grid  circuit,  the  energy  for  which  is  supplied  by  the  reactance 
voltage  of  the  coil  L-3.  Marked  amplifications  are  thus  secured. 


PART  III 

CASCADE    AMPLIFICATION    BY    THE 
VACUUM   VALVE 

19.  IN  GENERAL. —  In  the  preceding  chapter  we  have  described 
two  general  methods  by  which  damped  oscillations  in  radio  telegraphy 
can  be  detected  by  the  three-electrode  valve. 

In  the  first  method : 

(1)  The  potential  of  the  grid  in  respect  to  the  filament  may  be  main- 

tained so  that  a  positive  charge  impressed  upon  the  grid  will 
cause  a  large  increase  in  the  plate  current,  and  a  negative 
charge  will  cause  a  small  decrease  of  the  plate  current.  Or 
at  other  points  on  the  characteristic  curve,  the  reverse  effect 
may  be  obtained.  In  either  case,  the  effect  over  the  duration 
of  a  wave  train  is  to  impulse  the  telephone  receiver  once  for 
each  group  of  incoming  oscillations.  In  other  words,  what 
amounts  to  a  rectified  current  flows  in  the  plate  circuit. 

In  the  second  method : 

(2)  The  plate  current  is  made  to  vary  at  an  audio  frequency  by  placing 

a  condenser  in  series  with  the  grid.  The  valve  action  between 
the  grid  and  filament  rectifies  the  incoming  groups  of  radio 
frequency  oscillations,  and  the  grid  condenser  receives  a  uni- 
directional charge  over  the  duration  of  a  wave  train.  As  the 
charge  builds  up,  the  plate  current  decreases,  but  upon  the 
termination  of  a  group  of  oscillations  the  charge  leaks  out 
the  grid  condenser  and  the  plate  current  returns  to  normal 
value. 

The  sensitiveness  of  the  valve  in  the  second  case  is  determined  by 
its  construction,  i.e.,  the  size  of  the  grid,  and  the  spacing  between  it 
and  the  filament  or  the  plate.  If  the  construction  is  correct,  the  proper 
operating  characteristic  is  found  by  adjusting  the  filament  temperature 
and  the  voltage  of  the  plate  battery  until  maximum  response  is  secured. 
Reference  is  made  to  the  diagram  of  Figure.  ^6. 

Usually  with  this  circuit,  when  the  filament  is  rendered  incandes- 
cent the  potential  of  the  grid  will  be  somewhere  near  point  B,  Figure  22. 

51 


52  Vacuum  Tubes  In  Wireless  Communication 

In  other  words,  since  in  the  diagram  of  Figure  26  the  potential 
of  the  grid  in  respect  to  the  filament  cannot  be  definitely  fixed  by  a 
grid  battery,  the  operator  must  select  some  combination  of  filament 
temperature  and  plate  voltage  that  will  give  the  correct  operating  char- 
acteristic, for  maximum  response  in  the  head  telephone.  This  adjust- 
ment generally  is  not  difficult  to  find  with  a  properly  constructed  valve, 
but  in  event  that  the  valve  is  improperly  constructed  it  will  not  in  this 
circuit  act  efficiently  as  a  detector  of  incoming  oscillations. 

In  addition  to  (1)  and  (2)  we  have  established  another  fact: 

(3)  By  tuning  the  plate   circuit  to  the   radio  frequency   oscillations 

further  amplification  of  the  incoming  signal  is  secured. 

In  addition  to  (1),  (2)  and  (3)  we  must  remember  the  fundamental 
actions  of  the  valve  connected  as  in  Figure  24,  i.e., 

(4)  The  incoming  radio  frequency  currents  are  repeated  in  the  plate 

circuit,  but  with  distortion,  so  that  a  rectified  current  impulses 
the  telephone  diaphragm. 

Keeping  all  these  facts  before  us  it  is  obvious  that  the  valve  may 
be  connected  in  a  number'  of  ways  for  further  amplification  of  incoming- 
radio  signals  (damped  oscillations).  There  are  two  general  methods  by 
which  amplification  can  be  secured : 

(5)  In  the  reception  of  damped  oscillations  either  the  resultant  radio 

or  audio  frequency  component  of  the  plate  current  of  the  first 
valve  can  be  impressed  upon  the  filament  and  grid  or  input 
circuit  of  a  second  valve  and  again  amplified. 

(6)  Either  the   resulting  radio  or  audio  frequency  currents  or  both 

simultaneously  can  be  impressed  upon  the  grid  of  the  same 
valve  and  thus  re-enforced,  i.  e.,  regenerative  amplification  se- 
•         cured. 

We  shall  show  in  Part  VI. 

(7)  That  the   valve  can  be  used  to   generate  undamped  oscillations 

for  the  production  of  the  heterodyne  effect,  i.e.,  beat  re- 
ception; 

(8)  and  that  the  combined  operations  of  generation,  amplification  and 

beat  reception  can  be  performed  in  a  single  vacuum  tube 
simultaneously. 

We  first  shall  consider  the  circuits  and  functioning  of  the  cascade 
radio-frequency  amplifier. 

20.  CASCADE    RADIO    FREQUENCY    AMPLIFIER.— In    a 

circuit  where  the  radio  or  audio  frequency  component  of  the  plate  cur- 
rent of  one  valve  is  impressed  upon  the  grid  and  filament  of  the  second 
valve,  that  is  where  the  o.utput  circuit  of  the  first  valve  is  coupled  to 
the  input  circuit  of  the  second  valve  and  so  on,  the  valves  are  said  to 
operate  in  cascade.  In  practice,  as  many  as  six  valves  have  been  thus 


Vacuum  Tubes  In  Wireless  Communication  53 

used,  but  three  usually  suffice  for  practical  operating  conditions,  and 
little  gain  usually  results  from  employing  more. 

The  complete  circuits  of  a  cascade  radio  frequency  amplifying  sys- 
tem are  shown  in  Figure  29  where  a  coupling  transformer  M  with  the 
windings  PA  and  8  serves  to  impress  the  radio  frequency  component  of 
the  plate  circuit  of  the  first  valve  upon  the  grid  and  filament  of  the 
second  valve,  that  is  the  output  circuit  of  the  first  tube  is  coupled  to  the 
input  circuit  of  the  next  tube. 

The  antenna  circuit  of  this  diagram  embraces  the  antenna  loading 
inductance  L,  the  primary  winding  of  the  receiving  transformer  L-l, 
and  the  short  wave  variable  condenser  (7-8.  The  secondary  or  grid 
circuit  comprises  the  secondary  coil  L-2,  the  secondary  loading  induct- 
ance L-3,  the  shunt  secondary  condenser  (7-1,  and  the  fixed  condenser  (7 
shunting  po@tentiom.eter  P-l  which  has  resistance  of  about  400  or  500 
ohms. 

B-3  is  a  battery  from  three  to  twenty  volts.  5-1  is  the  usual  filament 
battery,  and  B-2,  the  high  voltage  battery  of  the  plate  circuit  of  the 
first  valve.  The  plate  circuit  also  includes  the  radio  frequency  circuit 
P-4,  CA ;  P-4  being  the  primary  of  a  radio  frequency  transformer  which 
acts  inductively  upon  winding  8.  LA  is  the  loading  inductance  for  the 
grid  circuit  of  the  second  valve  and  8  is  the  secondary  coil.  (7-5  is  the 
shunt  secondary  condenser.  The  condenser  C-6  performs  the  function 
of  (7  in  the  first  valve.  The  plate  circuit  of  the  second  valve  comprises 
the  battery  B-5,  the  head  telephone  P-3,  and  the  shunt  condenser  (7-7. 

The  operation  of  this  system  is  based  upon  the  phenomenon  shown 
by  the  characteristic  curve  in  Figure  22  which,  for  convenience,  is 
reproduced  in  Figure  30. 

Assume,  for  example,  that  the  potentiometer  P-l  of  valve  No  1  is 
adjusted  so  that  the  grid  potential  with  no  signals  in  the  receiving  sys- 
tem is  approximately  at  the  point  A  on  the  lower  bend  of  the  curve. 
Suppose  that  the  incoming  signals  are  relatively  weak,  i.e.,  of  just  suffi- 
cient strength  to  increase  and  decrease  the  grid  potential  a  small  frac- 
tion of  a  volt.  Then  as  the  curve  shows,  an  equal  increase  and  decrease 
of  the  plate  current  is  secured.  This  variation  takes  place  at  a  radio 
frequency,  and  consequently  no  response  would  be  secured  from  the 
telephone  diaphragm  if  it  were  connected  in  the  plate  circuit  of  this 
valve. 

On  the  other  hand,  if  the  signalling  E.M.F.  is  rather  strong,  then  it 
will  be  seen  from  the  shape  of  the  lower  bend  of  the  curve  that  the 
positive  half  of  the  incoming  cycle  will  produce  a  relatively  large  in- 
crease in  the  plate  current  and  the  negative  half  a  relatively  small 
decrease.  Therefore,  what  amounts,  in  effect,  to  a  rectified  current 


54 


Vacuum  Tubes  In  Wireless  Communication 


(BoOOQQJ 


g& 

•—  - 

O5      O   <!> 

01  flS 

O)      O   OJ 


bfi      §   c 

fe  S?? 


8  o 


Vacuum  Tubes  In  Wireless  Communication 


55 


would  flow  through  the  head  telephone,  P-3  (if  it  were  connected  in 
the  plate  circuit  of  the  first  valve).  But  if  the  incoming  signal  is 
too  weak  to  obtain  an  assymetrical  relaying  effect  in  the  plate  circuit  of 
the  first  valve,  response  can  only  be  obtained  in  the  receiving  telephone 
by  amplifying  the  plate  oscillations  of  the  first  valve  through  the  medium 
of  the  second  valve.  Then  if  the  second  valve  is  adjusted  to  the  proper 
operating  characteristic,  the  amplified  incoming  oscillations  impressed 
upon  its  grid  and  filament  will  cause  a  rectified  current,  that  is,  a  cur- 
rent which  increases  to  a  greater  degree  than  it  decreases  or  vice  versa, 
to  charge  the  telephone  condenser,  (7-7.  The  latter  discharges  into  the 
head  telephone  P-3  in  one  direction. 


PLATE  CURRENT 
k  — 

Ul  0 

D 

*' 

- 

CL*' 

— 

XJ 

b1^ 

t 

/ 

j 

/ 

> 

B 

y 

/ 

A 

/ 

— 

^ 

-; 

A 

0 

<* 

—  — 

^ 

•^ 

10                             -5 

0                             +5                                  K 
GRID    POTENTIAL 

Figure  30 

Grid-potential  plate-current  curve  of  the  three-electrode 
oscillation  valve. 


(a)  Curves  of  the  Cascade  Amplifier.  Diagrammatically,  the  am- 
plifying action  of  the  cascade  radio  frequency  system  can  be  shown  by 
the  series  of  curves,  Figure  31, — where  the  group  0-1  indicates  the  radio 
frequency  variation  of  the  continuous  plate  current  in  the  first  valve; 
the  group  0-2,  the  oscillating  E.M.F.  impressed  upon  the  grid  of  the 
second  valve,  and  the  group  0-3,  the  repeated  plate  current  in  valve  No. 
2.  The  average  effect  of  the  uni-directional  plate  current  pulses  as  far 
as  the  head  telephone  diaphragm  is  concerned  is  indicated  by  the  line 
0-4.  This  variation  of  the  plate  current  takes  place  at  an  audio  fre- 
quency, i.e.,  it  follows  the  spark  at  the  transmitter*. 

In  this  system,  the  plate  circuit  of  valve  No.  1  may  be  tuned  or 
untuned.  It  is  essential,  however,  that  the  grid  circuit  of  the  second 


*If  the  last  or  detecting  tube  is  adjusted  somewhat  near  the  upper  bend  of 
the  characteristic  curve,  the  telephone  current  decreases  for  each  group  of  in- 
coming radio  frequency  oscillations. 


56 


Vacuum  Tubes  In  Wireless  Communication 


valve  be  accurately  adjusted  to  the  radio  frequency  component  of  the 
plate  current  of  the  first  valve.  This  is  accomplished  by  the  variable 
shunt  condenser  (7-5. 

The  plate  circuit  of  valve  No.  1  may  be  tuned  to  resonance  with 
its  radio  frequency  component  by  means  of  the  variable  condenser  (7-4. 
If  condenser  (7-4  in  shunt  to  the  primary  winding  P-4  gives  that  circuit 


o-i 


PLATE  CURRENT 
VALVE  N°.  I 


GRID   POTENTIAL 
VALVE 


PLATE   CURRENT 
VALVE  N«2 


0-5 


APPROXIMATE  TELEPHONE 
CURRENT  VALVE  N?2 


0-4 


Figure  31 

Graphs   showing  the  functioning  of  the   cascade   radio   frequency 
amplifier. 

a  frequency  of  oscillation  equal  to  that  of  the  radio  frequency  com- 
ponent of  the  continuous  plate  current,  then  a  well  defined  resonance 
phenomenon  takes  place  and  the  amplitude  of  the  plate  circuit  oscilla- 
tions is  increased.  Tuning  of  the  plate  circuit  is  more  effective  in  prac- 
tice, when  the  incoming  oscillations  (induced  in  the  receiving  antenna 
A)  are  comparatively  weak. 


Vacuum  Tubes  In  Wireless  Communication  57 

In  the  operation  of  the  apparatus  shown  in  Figure  29,  some  energy 
is  withdrawn  from  the  circuits  of  the  first  valve  by  the  second  valve 
through  the  coupling  M,  and  in  consequence,  as  the  coupling  is  altered 
re-adjustments  of  the  various  elements  of  the  complete  circuit  ordinarily 
must  follow. 

In  order  to  secure  amplification  from  the  apparatus  of  Figure  29, 
the  potentiometer  P-l  should  be  adjusted  so  that  the  grid  potential  will 
have  some  value  along  the  straight  part  of  the  curve,  say  near  the  point 
B,  Figure  30;  but  the  grid  potential  of  the  second  valve  must  be  ad- 
justed (by  potentiometer  P-2)  to  take  full  advantage  of  the  assymetrical 
conductivity  of  the  valve,  i.e.,  to  point  A,  Figure  30.  Then  condenser 
(7-7  will  receive  a  uni-directional  charge  over  the  duration  of  each  wave 
train  and  its  discharge  through  the  telephone  P-3  will  be  practically 
aperiodic,  that  is,  in  one  direction. 

Instead  of  adjusting  the  potential  of  the  grid  of  value  No.  2,  Figure 
29,  to  take  advantage  of  its  assymetrical  properties,  we  may  insert  a 
grid  condenser  such  as  shown  at  (7  in  Figure  26.  The  circuit  then  func- 
tions identically  as  explained  in  connection  with  that  diagram,  that  is, 
the  incoming  oscillations  are  rectified,  and  a  charge  accumulates  in  the 
grid  condenser  over  the  duration  of  a  wave  train.  As  the  charge  in- 
creases, the  plate  current  of  valve  No.  2  gradually  decreases.  At  the 
termination  of  a  group  of  oscillations  the  charge  leaks  out  the  grid 
condenser  either  through  the  valve  or  through  a  special  leak  resistance 
(shunted  about  the  grid  condenser  or  connected  from  grid  to  filament), 
whereupon  the  plate  current  (battery  5-5)  returns  to  normal  strength. 
This  action,  as  has  already  been  explained,  is  repeated  for  each  spark 
discharge  at  the  transmitter. 

21.  CASCADE     AUDIO     FREQUENCY     AMPLIFIER.— We 

have  pointed  out  in  paragraph  17  (and  shown  the  connections  therefor 
in  the  diagram  of  Figure  26)  how  the  successive  groups  of  incoming 
(damped)  oscillations  may  be  rectified  and  stored  up  in  a  grid  Con- 
denser, and  how,  during  the  piling  up  of  this  charge  the  plate  current 
decreases.  At  the  termination  of  the  incoming  wave  train  the  charge 
leaks  out  of  the  condenser  and  the  plate  current  returns  to  normal  value. 
This  variation  of  the  continuous  current  in  the  plate  circuit  is  termed 
the  audio  frequency  component  of  the  plate  current. 

The  audio  frequency  component  may  be  amplified  through  the 
medium  of  a  second  valve.  One  method  is  shown  in  the  diagram  of 
Figure  32,  wherein  the  plate  circuit  of  the  first  valve  and  the  grid 
circuit  of  the  second  valve  are  coupled  inductively  through  the  iron 
core  transformer  M.  The  primary  and  secondary  coils  of  M  consist 


58 


Vacuum  Tubes  In  Wireless  Communication 


Vacuum  Tubes  In  Wireless  Communication  59 

of  several  thousand  ampere  turns  of  relatively  fine  wire  such  as  No. 
32  or  No.  34  B  and  8  wound  over  a  common  iron  core,  the  inductance 
of  either  winding  amounting  to  a  henry*  or  more.  Condensers  (7-3  and 
(7-6  serve  to  tune  the  primary  and  secondary  circuits  of  M  to  the  desired 
audio  frequency  of  300  to  1,000  cycles,  although  they  may  be  omitted 
with  good  results. 

In  order  that  valve  No.  2  may  be  adjusted  for  maximum  amplifi- 
cation, battery  B-6  and  potentiometer  P-l  are  included  in  the  grid 
circuit. 

The  apparatus  of  Figure  32  functions  as  follows:  The  successive 
groups  of  incoming  oscillations  are  converted  to  audio  frequency  varia- 
tions of  the  plate  current  (B-2)  through  the  charge  and  discharge  of 
the  grid  condenser.  This  component  of  the  plate  current  is  impressed 
upon  the  grid  circuit  of  the  second  valve  through  the  coupling  M. 
Through  the  medium  of  the  grid  potentiometer  P-l  the  second  tube  is 
adjusted  for  the  best  amplification  of  the  impressed  audio  frequency 
current.  The  condenser  C  of  valve  No.  1  may  be  dispensed  with  and  the 
valve  adjusted  for  assymetrical  relaying  by  providing  a  special  grid 
battery,  as  with  valve  No.  2. 

In  practice,  a  simpler  circuitf  for  audio  frequency  amplification 
than  that  shown  in  Figure  32  has  been  employed.  Generally,  three 
valves  are  connected  in  cascade,  being  coupled  together  through  simple 
iron  core  transformers  between  the  output  and  input  circuits  of  suc- 
cessive valves.  No  attempt  is  made  to  tune  these  transformers  to  the 
desired  frequency. 

Although  the  apparatus  shown  in  Figure  32  is  designed  primarily 
for  the  amplification  of  audio  frequencies,  some  of  the  radio  frequency 
energy  of  the  plate  circuit  of  the  first  valve  is  impressed  upon  the  grid 
circuit  of  the  second  valve  through  the  electrostatic  capacity  of  the 
windings  of  M,  and  detected  in  the  last  valve.  It  may  then  be  amplified 
by  a  regenerative  circuit  or  a  radio  frequency  amplification  circuit. 

A  single  coil,  i.e.,  an  auto-transformer,  may  be  substituted  for  the 
transformer  M. 


*Some  transformers  of  this  type  have  primary  inductance  of  15  henries  and 
secondary  inductance  of  approximately  90  henries. 

fin  one  type  of  commercial  cascade  amplifier,  an  oddly  constructed  trans- 
former is  employed  between  the  plate  and  the  grid  circuit  of  the  successive 
valves.  The  primary  winding  of  the  audio  frequency  transformer  is  inserted  in 
series  with  the  plate  circuit  in  the  usual  manner,  but  the  secondary  winding  is 
left  open-circuited,  that  is  to  say,  one  terminal  is  connected  to  the  grid  of  the 
valve  but  the  other  terminal  remains  free.  By  this  manner  of  coupling  it  is 
possible  to  employ  a  single  battery  to  light  the  filaments  of  all  valves.  Arm- 
strong mentions  that  leaving  the  secondary  circuit  open  permits  the  grid  to 
assume  a  potential  suitable  to  amplification. 


60 


Vacuum  Tubes  In  Wireless  Communication 


o  ca 


Vacuum  Tubes  In  Wireless  Communication  61 

22.  COMBINED  RADIO  AND  AUDIO  FREQUENCY 
CASCADE  AMPLIFIER. — By  means  of  the  connections  shown  in 
Figure  33,  the  radio  and  audio  frequency  components  of  the  plate  or 
output  circuit  of  one  valve  may  be  amplified  simultaneously  through  a 
second  valve. 

In  this  system,  the  audio  frequency  component  of  the  plate  circuit 
is  impressed  upon  the  grid  circuit  of  the  second  valve  through  trans- 
former M.  Condenser  0-3  serves  as  a  by-pass  for  the  radio  frequency 
currents  around  the  audio  frequency  inductance  L-5.  Condenser  0-2 
serves  to  tune  the  plate  circuit  to  the  incoming  oscillations  (the  radio 
frequency  component  of  the  plate  current)  and  0-5  tunes  the  grid  cir- 
cuit of  the  second  valve  to  the  same  frequency,  the  radio  frequency  cur- 
rent of  the  plate  circuit  being  impressed  upon  the  grid  circuit  of  the 
second  valve  through  the  coupling  i-3,  LA.  Condenser  0-4  acts  to 
by-pass  the  radio  frequency  current  in  the  grid  circuit  of  the  second  valve 
around  the  audio  frequency  coil  L-6.  Potentiometer  P-2  shunting  bat- 
tery B-6  enables  the  operator  to  obtain  the  best  relaying  characteristic 
of  valve  No.  2. 

As  usual,  a  grid  condenser  can  be  inserted  in  the  circuit  of  valve 
No.  2,  and  the  relaying  action  obtained  without  the  potentiometer.  The 
battery  B-Q  is  preferably  shunted  by  a  condenser  of  fixed  capacity. 

At  first  sight,  the  circuits  shown  in  this  chapter  seem  complicated 
in  structure  and  difficult  of  adjustment,  but  it  should  be  understood 
that  if  the  tuner  is  designed  for  a  given  range  of  wave  lengths,  it  is 
not  necessary  to  re-adjust  each  element  of  the  circuit  to  select  a  new 
wave  length.  For  example,  the  apparatus  could  readily  be  designed  so 
that  the  principal  adjustments  for  resonance  are  obtained  at  the  primary 
coil,  the  secondary  condenser,  the  grid  circuit  condenser,  and  plate 
circuit  condenser.  A  slight  variation  of  the  grid  circuit  potentiometer 
is  necessary  for  the  louder  strength  of  signals. 


PART  IV 

THE  REGENERATIVE  VACUUM  VALVE 
AMPLIFIER 

23.  IN  GENERAL. — The  preceding  chapter  is  devoted  to  ampli- 
fication by  cascade  connection  of  a  number  of   valves.     This   section 
will  be  devoted  to  the  regenerative  system  of  amplification  whereby  iliu 
strength  of  the  incoming  radio  signals  is  increased  within  the  same 
bulb.     Experiments  with  such  circuits  were  first  leported  by  Captain 
Edwin  H.  Armstrong,  whose  researches  have  been  fully  described  in 
the  September  1915,  issue  of  "Proceedings  of  the  Institute  of  Radio 
Engineers. ' ' 

24.  PRELIMINARY  CONSIDERATIONS.— We    have    already 
shown  that  the  vacuum  valve  acts  as  a  repeater  of  radio  frequency 
currents,  i.e.,  if  an  oscillating  voltage  is  applied  to  the  grid  and  filament, 
the  plate  current  will  oscillate  at  the  same  frequency.     Also  through  the 
use  of  a  grid  condenser  during  the  reception  of  damped  oscillations  the 
plate  current  varies  simultaneously  at  an  audio  and  a  radio  frequency ; 
or  through  the  employment  of  a  grid  battery,  we  can  adjust  the  grid 
potential  so  that  the  increase  of  the  plate  current  for  each  incoming 
semi-cycle  will  exceed  the  decrease  or  vice  versa  and  what  in  effect 
amounts  to  a  rectified  current  flows  in  the  plate  circuit.     Careful  con- 
sideration of  these  fundamental  actions  will  assist  the  student  to  under- 
stand the  functioning  of  the  regenerative  receiver. 

We  shall  disclose  in  this  chaper  circuits  whereby  either  the  audio 
or  radio  frequency  component  of  the  plate  current  can  be  impressed 
upon  the  grid  of  the  same  valve  and  thus  reinforced.  The  circuits 
through  which  such  amplifications  are  obtained  are  known  as  "regenera- 
tive" or  "repeater"  circuits.  Abroad,  they  are  termed  "re-action" 
circuits. 

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Vacuum  Tubes  In  Wireless  Communication 


63 


25.  THE  PRINCIPLE  OF  REGENERATION.— We     have     al- 
ready set  forth  how  variation  of  the  grid  potential  of  the  three  electrode 
vacuum  valve  changes  the  strength  of  the  plate  current ;  and  that  a  very 
slight  change  in  the  grid  potential,  if  the  proper  bulb  characteristic  is 
secured,  results  in  a  relatively  large  change  in  the  plate  current. 

The  characteristic  curve  of  the  valve  indicates  that  if  by  any  means 
the  grid  potential  is  reinforced  above  and  below  the  maximum  value 
supplied  by  the  incoming  radio  signal,  a  still  greater  change  in  the 
telephone  current  is  bound  to  follow. 

Since,  during  the  reception  of  radio  signals  the  plate  current  varies 
at  the  frequency  of  the  incoming  oscillations  it  will  be  seen  that  if  this 
radio  frequency  component  of  the  plate  current  can  be  impressed  upon 
the  grid  circuit  in  synchronism  with  the  incoming  signals,  the  energy  of 
the  original  signal  will  be  increased,  i.e.,  regeneration  will  result. 

In  brief,  regenerative  coupling  is  secured  by  coupling  the  plate  and 
grid  circuits,  inductively,  conductively,  or  electrostatically.  Electrostatic 
coupling  may  be  furnished  by  the  tube  itself. 

26.  RADIO  FREQUENCY  REGENERATIVE  CIRCUIT.— 

A  system  for  the  regeneration  of  radio  frequencies  first  disclosed  by 
Captain  Edwin  H.  Armstrong  is  shown  (in  simple  form)  in  Figure  34. 
In  this  diagram,  the  plate  or  output  circuit  of  the  valve  is  coupled 


Figure  34  ^-^* ~~  P-l 

\ 

Simple   regenerative   receiver   for   the   amplification   of   radio   fre- 
quencies. 


to  its  grid  or  input  circuit  through  the  radio  frequency  transformer  M 
with  the  primary  and  secondary  windings  P  and  8  respectively.  (The 
radio  frequency  components  of  the  plate  current  are  the  cycles  of  Figure 
35  which  curve  is  partially  reproduced  from  Figure  27.) 


64 


Vacuum  Tubes  In  Wireless  Communication 


(a)  Phenomena  of  Regeneration.  Just  how  the  regenerative  sys- 
tem amplifies  the  incoming  signals  may  be  explained  more  in  detail,  as 
follows :  A  very  small  amount  of  the  energy  applied  to  the  grid  circuit 
will  release  a  considerably  greater  amount  of  energy  on  the  plate  circuit. 
Part  of  the  energy  liberated  in  the  plate  circuit  is  in  turn  impressed  upon 
the  grid  circuit  through  the  coupling  transformers  and  the  difference 
between  the  maximum  and  minimum  potential  difference  between  the 
grid  and  filament  accordingly  increased.  Increased  energy  is  then 
liberated  in  the  plate  circuit,  but  this  regenerative  process  cannot  con- 
tinue indefinitely  for  the  final  amplitude  of  the  regenerated  current  is 


Figure  35 

Radio    frequency    component    of   the    continuous 
plate  current. 


Figure  36 
Graphs  showing  the  phenomena  of  regeneration. 

governed  by  the  limitations  explained  in  paragraph  12.  Then  if  an 
oscillating  E.M.F.  is  impressed  upon  the  grid  it  is  repeated  in  the  plate 
circuit  and  through  coupling  to  the  grid  circuit  the  energy  of  the  original 
current  is  increased. 

The  effects  of  regenerative  coupling  may  be  shown  by  the  curves  A 
and  B,  Figure  36.    Curve  A  shows  the  amplitude  of  the  incoming  radio 


Vacuum  Tubes  In  Wireless  Communication  65 

frequency  currents,  and  curve  B  the  increased  number  of  oscillations 
due  to  regeneration.  If  the  coupling  of  the  regenerative  transformer, 
such  as  M  in  Figure  34,  is  very  close  the  system  will  be  set  into  self- 
oscillation  (at  a  frequency  determined  by  the  inductance  and  capacity 
of  the  circuit)  as  shown  in  the  curve  of  Figure  37 ;  but  this  is  a  condition 
of  affairs  not  yet  desired  in  the  circuits  under  consideration. 

(b)  Adjustment  of  Regenerative  Amplifier.  In  order  that  the 
maximum  strength  of  signals  may  be  obtained  from  a  spark  transmitter 
by  means  of  the  regenerative  system,  it  is  essential  that  the  coupling 
of  the  transformer  M*  be  very  carefully  adjusted,  because  (it  will  be 
recognized  from  the  curves  of  Figure  36 )  if  the  coupling  is  too  close,  the 


Figure  37 

Graph  showing  the  continuous  oscillations  resulting 
when  the  grid  and  plate  circuits  of  the  vacuum  tube 
are  closely  coupled. 

oscillations  for  each  spark  at  the  transmitter  will  not  decay  to  zero 
before  the  next  group  is  impressed  upon  the  valve  circuits.  Conse- 
quently, the  desired  change  of  current  through  the  head  telephone  will 
not  be  obtained.  It  must  be  kept  in  mind  here,  that  it  is  not  alone  the 
amount  of  current  flowing  through  the  head  telephone  which  produces 
the  greatest  strength  of  signals,  but  it  is  the  change  or  variation  in  the 
strength  of  current  as  well,  which  deflects  the  telephone  diaphragm. 

The  apparatus  shown  in  Figure  34  does  not  differ  materially  from 
that  of  the  tuned  circuit  for  the  vacuum  valve  shown  in  Figure  28,  for 
in  Figure  28  the  internal  capacity  of  the  tube  acts  as  the  regenerative 
coupling.  The  complete  functioning  of  the  regenerative  circuit  may  be 
summed  up  in  the  following  statements: 

(1)  The  incoming  oscillations  are  repeated  in  the  plate  circuit,  rein- 

forced through  coupling  to  the  grid  circuit,  causing  still 
greater  variation  of  the  grid  potential;  in  the  meanwhile, 
through  rectification,  a  charge  piles  up  in  the  grid  condenser 
which  is  negative  on  the  grid  side  of  the  condenser.  This 
partially  obstructs  the  flow  of  electrons  to  the  plate,  and 
thereby  reduces  the  strength  of  the  plate  current. 

(2)  At  the  termination  of  the  wave  train  the  charge  in  the  grid  con- 

denser leaks  off  through  the  shunt  resistance,  the  grid  returns 
to  normal  potential,  and  the  plate  current  returns  to  normal 
value. 


*The  statement  applies  equally  well  to  electrostatic  coupling  between  the 
grid  and  plate  circuits. 


66  Vacuum  Tubes  In  Wireless  Communication 

Amplifications  of  fifty  fold  are  thus  secured. 

In  practice,  the  best  operating  characteristic  of  the  regenerative 
circuit  of  figure  34  may  be  secured  in  the  following  manner : 

(1)  Close  the  circuit  from  B-l  through  filament  F.     Find  the  correct 

degree  of  incandescence  either  by  trial  or  by  a  small  ammeter 
connected  in  series  with  the  battery. 

(2)  Adjust  the  E.M.F.  of  battery  B-2  near  to  the  value  necessary  for 

the  correct  operating  characteristic  (which  may  be  deter- 
mined in  advance  in  the  laboratory  or  may  be  found  by 
experiment.) 

(3)  While  tuning  the  open  and  closed  circuits  of  the  receiving  tuner 

to  the  distant  transmitter  use  small  values  of  capacities  at 
C-l,  and  relatively  large  values  of  inductance  at  L-2. 

(4)  Adjust  the  coupling  of  regenerative  transformer  M  until  maximum 

strength  of  signals  is  secured. 

The  function  of  condenser  (7-2  is  to  by-pass  the  radio  frequency 
component  of  the  plate  current  around  the  head  telephone.  Its  capacity 
is  generally  fixed.  The  apparatus  will  function  without  this  condenser, 
the  required  capacity  being  found  in  the  parallel  cords  of  the  head 
telephone. 

27.  TUNED  PLATE  CIRCUITS.— We  have  illustrated  in  Figure 
28,  (see  paragraph  18),  the  tuned  plate  circuit  of  Armstrong.  It  was 
mentioned  that  amplification  of  the  incoming  signal  is  thus  secured. 

Either  the  radio  or  audio  frequency  component  of  the  plate  current 
may  be  amplified  by  proper  tuning. 

The  explanation  given  by  Armstrong  for  the  operation  of  this  cir- 
cuit is  somewhat  as  follows : 

When  the  grid  circuit  is  not  in  a  state  of  oscillation,  the  potential 
difference  between  the  plate  and  filament  will  be  nearly  that  of  the  bat- 
tery B-2,  but  during  the  reception  of  radio  oscillations,  the  potential 
difference  between  the  plate  and  filament  varies  accordingly  as  the 
reactance  voltage  of  the  coil  L-3  assists  or  opposes  the  voltage  of  the 
local  battery  B-2. 

Then  if  the  current  in  the  plate  circuit  decreases,  the  reactance 
voltage  (due  to  the  collapsing  lines  of  force  about  L-3)  will  be  in  the 
same  direction  as  the  voltage  of  the  battery  B-2,  and  therefore  there  will 
be  an  increase  of  potential  difference  between  the  plate  and  filament. 
But  when  the  current  from  battery  B-2  increases,  the  reactance  voltage 
of  L-3  will  oppose  that  of  the  battery  decreasing  the  potential  difference 
between  the  plate  and  filament. 

It  is  clear  from  the  preceding  explanations  regarding  the  function- 
ing of  the  three-electrode  tube,  that  when  a  negative  charge  is  impressed 
upon  the  grid,  the  plate  current  (the  circuit  of  B-2)  decreases,  but  when 


Vacuum  Tubes  In  Wireless  Communication  67 

a  positive  charge  is  applied  to  the  grid,  the  plate  current  increases. 
Hence,  when  the  filament  and  grid  are  connected  to  a  source  of  radio 
frequency  oscillations,  the  current  in  the  plate  circuit  varies  as  the 
applied  frequency. 

Then,  when  a  negative  charge  is  placed  on  the  grid,  the  plate  cur- 
rent is  reduced  and  the  reactance  voltage  of  the  coil  L-3  acts  in  the  same 
direction  as  B-2  and  increases  the  potential  difference  between  the  plate 
and  the  filament  drawing  more  electrons  out  on  the  grid.  This  increases 
the  charge  in  the  condenser  formed  by  the  plate  and  grid,  and  the 
energy  for  this  increased  charge  is  furnished  by  the  inductance  L-3  as 
the  battery  current  of  B-2,  decreases. 

The  increased  negative  charge  on  the  grid  tends  to  produce  a  still 
further  decrease  in  the  plate  current  which  causes  a  still  further  dis- 
charge of  energy  from  the  plate  inductance  L-3  into  the  grid  circuit. 

But  when  a  positive  charge  is  placed  on  the  grid  the  plate  current 
is  increased  and  the  reactance  voltage  of  L-3  opposes  the  voltage  of  the 
battery  B-2.  This  reduces  the  potential  difference  between  the  grid  and 
plate,  and  therefore  a  part  of  the  energy  stored  up  in  the  condenser 
formed  by  the  grid  and  plate  is  given  back  to  the  plate  inductance. 

It  is  at  this  moment  during  the  cycle  that  electrons  are  drawn  into 
the  grid  in  accordance  with  the  valve  action  and  during  a  group  of  in- 
coming oscillations  a  charge  is  gradually  trapped  in  the  condenser  C-2 
which  at  the  termination  of  a  wave  train  leaks  off  the  grid  exerting  the 
usual  relaying  action  on  the  plate  current.  This  relaying  action,  during 
the  reception  of  damped  oscillations,  occurs  at  an  audio  frequency. 

We  then  see  the  necessity  for  resonance  between  the  plate  and  grid 
circuits.  In  order  that  the  transference  of  energy  from  the  plate  to  the 
grid  circuit  may  take  place  in  synchronism  with  the  incoming  oscillations, 
the  circuit  L-3  and  the  condenser  formed  by  the  self-capacity  of  the 
valve  must  possess  substantially  the  same  natural  frequency.  Then 
energy  will  be  transferred  from  the  plate  to  the  grid  circuit  at  the  proper 
time  to  increase  the  final  amplitude  of  the  incoming  oscillations,  i.e.,  re- 
generation will  result.  This  is  substantially,  electrostatic  regenerative 
coupling. 

In  order  that  the  tuning  of  the  plate  circuit  may  be  effective  at  the 
lower  frequencies  corresponding  to  the  longer  wave  lengths,  the  coil  Z/-3 
should  be  shunted  by  the  condenser  (7-3  as  shown  by  the  dotted  lines. 
Armstrong  states  that  the  circuit  will  then  function  at  wave  lengths 
in  excess  of  10,000  meters. 

Other  methods  for  tuning  the  plate  circuit  are  shown  in  Figures  38, 
39,  and  40.  In  Figure  38,  resonance  is  secured  by  the  variable  induct- 


68 


Vacuum  Tubes  In  Wireless  Communication 


ance  L-3  which  at  the  lower  oscillation  frequencies  is  shunted  by  con- 
denser C-3  (of  approximately  .001  microfarad  capacity).  Condenser  C-2 
acts  as  a  by-pass  for  the  radio  frequency  currents  around  the  head  tele- 
phone and  battery.  Its  capacity  may  be  fixed.  The  primary  coil  of  the 
regenerative  coupler  P  is  a  part  of  the  tuning  circuit. 


c-s 


P-I 

Figure  38 

Armstrong's  regenerative  system  (with  a  tuned  plate  circuit)  for 
the  amplification  of  radio  frequencies. 


Figure  39  '»     C-z 

Second  method  of  tuning  the  plate  circuit  in  a  regenerative  system. 

In  Figure  39  the  plate  circuit  is  tuned  by  inductance  P  and  eon- 
denser  C-3,  which  are  proportioned  for  resonance  with  the  incoming 
signal. 

A  third  method  of  tuning  the  plate  circuit  is  shown  in  Figure  40, 
wherein  battery  B-2  and  head  telephone  P-l  are  shunted  by  condenser 
C-3,  and  coils  L-3  and  P.  Resonance  may  be  established  either  by  varia- 


Vacuum  Tubes  In  Wireless  Communication 


69 


tion  of  C-3  or  L-3.  Generally,  in  practice,  L-3  is  fixed,  tuning  being  ac- 
complished through  condenser  (7-3.  This  tuned  plate  circuit  is  essentially 
the  "  X  "  circuit  developed  by  Weagant  which  is  described  in  paragraphs 
44  and  45. 

28.  AUDIO  FREQUENCY  REGENERATIVE  SYSTEM.— 

The  audio  frequency  component  of  the  plate  current  can  be  re-enforced 
to  amplify  the  incoming  signal  by  the  regenerative  transformer  M ,  Figure 
41.  Windings  P  and  S  have  inductance  of  approximately  one  henry  each 
or  more.  Condenser  C-2  serves  to  by-pass  the  radio  frequency  current 
of  the  incoming  signal,  and  if  of  variable  capacity,  permits  circuit  S,  C-2 
to  be  tuned  to  the  audio  frequency  component  of  the  plate  circuit. 


C-3 


Figure  40 
Third  method  of  tuning  the  plate  circuit  in  a  regenerative  system. 

The  circuit,  in  brief,  functions  as  follows:  As  the  incoming  oscilla- 
tions are  rectified  and  trapped  in  the  grid  condenser  G,  the  plate  current 
is  gradually  reduced  (due  to  the  increasing  negative  potential  of  the 
grid).  The  resulting  reduction  and  subsequent  increase  of  the  con- 
tinuous plate  current  in  turn  causes  winding  P  to  act  inductively  upon  S, 
charging  the  condenser  C-2  which  discharges  into  the  grid  circuit  caus- 
ing still  greater  variation  of  the  plate  current.  Audio  frequency  currents 
are  thus  amplified. 

29.  COMBINED  AUDIO  AND  RADIO  FREQUENCY  RE- 
GENERATIVE SYSTEM.— We  shall  now  consider  a  regenerative  sys- 
tem through  which  both  the  audio  and  radio  frequency  components  of  the 
plate  current  may  be  amplified  through  the  same  bulb.  The  working  of 
this  system  will  be  readily  understood  by  one  who  has  studied  the  previous 
regenerative  systems. 

The  circuit  is  shown  in  Figure  42.    Here  M-2  is  a  radio  frequency 


70 


Vacuum  Tubes  In  Wireless  Communication 


transformer  through  which  the  radio  frequency  component  of  the  plate 
current  is  re-enforced  through  the  grid  G. 

Transformer  M-l  is  an  iron-core  audio  frequency  coupling  for  ampli- 
fying the  audio  frequency  component  of  the  plate  circuit,  the  primary 
P  being  shunted  by  (7-3,  and  the  secondary  S  by  condenser  (7-2. 


R-I-- 


/wwvwv\ 

Ik 


Figure  41 
The  circuits  of  an  audio  frequency  regenerative  amplifier. 

Condenser  (7-3  tunes  circuit  P,  (7-3  to  the  desired  audio  frequency 
and  condenser  (7-2  performs  the  double  function  of  providing  a  path  for 
the  radio  frequency  current  through  the  grid  circuit  and  tuning  circuit 
(7-2,  S  to  the  audio  frequency  component.  Condenser  (7-4  provides  a  path 
for  the  radio  frequency  current  around  the  head  telephones  P-2.  Induct- 
ance L-3  tunes  the  plate  circuit  to  the  incoming  signal. 

This  circuit  was  first  published  by  Armstrong,  who  declares  that 
with  relatively  weak  incoming  signals  a  total  amplification  of  100  is  ob- 
tained. Some  difficulty  is  experienced  in  keeping  this  system  in  stable 
operation. 

30.  ELECTRO-STATIC  AND  DIRECT  MAGNETIC  COUPL- 
ING.—  Additional  regenerative  circuits*  devised  by  Captain  Armstrong 
are  shown  in  Figures  43  and  44.  The  grid  and  plate  circuits  in  Figure 
43  are  coupled  electrostatically  through  condenser  (7-1  and  the  coil  M 
of  audio  frequency  dimensions  completes  the  circuit  for  the  plate  current. 

According  to  Armstrong  the  apparatus  shown  in  Figure  43  func- 
tions as  follows :  When  a  positive  charge  is  placed  on  the  grid  an  increase 
in  the  plate  current  results,  the  alternating  component  of  the  wing  cur- 


*See  Proceedings  of  the  Institute  of  Radio  Engineers,  Sept.,  1915,  Vol.  3, 
No.  3. 


Vacuum  Tubes  In  Wireless  Communication 


71 


sg. 


72 


Vacuum  Tubes  In  Wireless  Communication 


rent  charging  the  condenser  0-1,  and  the  sum  of  the  currents  through  C-l 
and  M  equalling  the  current  through  the  valve.  When  a  negative  charge 
is  placed  upon  the  grid,  the  current  through  the  valve  is  reduced  and 
the  coil  M  discharges  into  condenser  (7-1,  charging  it  in  the  opposite 
way  to  that  caused  by  the  increase  of  the  plate  current.  In  either  case 
C-l  discharges  through  the  grid  circuit  re-enforcing  the  oscillations 
therein. 


Figure  43 

Showing  the  circuits  of  Armstrong's  regenerative  system  wherein 
the  plate  and  grid  circuits  are  electrostatically  coupled  through  a 
condenser  (C-l). 


Figure  43a 
Ultra-audion  circuit  for  regenerative  amplification. 

A  modification  of  the  arrangement  of  Figure  43  is  in  use,  known 
under  the  trade  name  of  "ultra  audion."  The  circuit  arrangement  is 
shown  in  Figure  43a.  Its  operation  is  the  same  as  that  of  Figure  43 
except  that  radio  frequencies  alone  are  regeneratively  amplified.  It  will 
be  seen  that  this  mode  of  using  the  coupling  condenser  C-l  in  shunt  to 


Vacuum  Tubes  In  Wireless  Communication 


73 


both  the  battery  and  the  telephones  results  in  apparently  connecting  the 
secondary  oscillation  circuit  between  the  grid  and  plate.  This  is  more 
apparent  than  real,  however,  as  it  is  effectively  connected  to  the  filament 
through  the  coupling  condenser  0-1. 


Figure  44 

Direct  magnetic  coupling  between  the  plate  and  grid  circuits  of 
the  vacuum  tube. 


The  circuit  of  Figure  44  functions  like  Figure  34,  conductive  regen- 
erative coupling  (M)  instead  of  inductive  coupling  being  used. 

31.  THE  "REACTION"  CIRCUITS  OF  FRANKLIN  AND 
MARCONI. — It  is  customary  in  England  to  speak  of  regenerative  cir- 
cuits as  "reaction"  circuits.  The  term,  in  general,  refers  to  a  valve 
circuit  in  which  the  grid  and  plate  circuits  are  coupled  magnetically  or 
electrostatically. 

An  interesting  circuit  developed  by  Franklin,  of  Marconi's  Wireless 
Telegraph  Company,  Ltd.,  is  shown  in  Figure  45.  It  is  to  be  noted  in 
this  diagram  that  the  grid  and  plate  circuits  are  coupled  at  LA  and  L-5 ; 
that  the  plate  circuit  is  tuned  by  L-5,  L-6,  0-1 ;  and  that  the  secondary 
circuit  of  the  receiving  tuner  L-3,  L-4,  0,  is  tuned  to  resonance  with 
the  incoming  signal  by  variable  condenser  0.  Furthermore,  the  potential 
of  the  grid  in  respect  to  the  filament  is  adjusted  by  battery  JB-3  shunted 
by  potentiometer  P-l. 

So  far,  the  circuit  does  not  differ  from  previous  regenerative  systems. 
It  will  be  seen,  however,  that  the  plate  tuning  and  coupling  elements 
L-5,  L-6,  and  0-1  are  shunted  by  a  crystal  rectifier  circuit  D,  P-2,  0-2, 
P-3,  5-4,  the  circuit  being  similar  to  the  closed  or  secondary  circuit  of  the 
usual  receiving  tuner. 


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The  advantage  of  a  circuit  of  this  kind  is  as  follows :  Assume  that 
the  incoming  signals  are  too  weak  to  give  response  in  a  simple  crystal 
rectifying  circuit :  then  they  may  be  amplified  by  the  vacuum  tube  in 
the  usual  manner.  Further  amplification  may  be  secured  through  the 
reaction  coupling  L-4,  L-5.  Suppose,  however,  the  output  current  of  the 
valve  increases  and  decreases  by  equal  amounts  so  that  a  telephone  if 
placed  in  the  plate  circuit  would  give  no  response.  (This  has  been 
explained  in  paragraph  20).  Then  the  radio  frequency  component  of 
the  circuit  may  be  detected  by  simple  rectification  through  the  crystal  D; 
that  is,  if  the  amplitude  of  the  positive  or  negative  halves  of  the  plate 
current  have  equal  value,  a  telephone  connected  in  series  will  give  no 
response,  but  by  connecting  the  crystal  rectifier  as  shown  in  the  diagram, 
the  radio  frequency  component  is  rectified  and  made  audible  in  the  head 
telephone. 

Another  advantage  over  the  simple  crystal  rectifier  is  thus  secured, 
viz.,  increased  selectivity  due  to  the  following  phenomenon.  Normally, 
let  us  say  the  incoming  signal  is  highly  damped,  consisting  of  but  a  few 
oscillations:  the  additional  energy  liberated  by  the  plate  circuit  through 
the  reaction  coupling  actually  increases  the  number  of  oscillations  and 
therefore  the  circuit  acts  like  one  in  which  the  damping  has  been  reduced, 
i.e.,  it  permits  better  discrimination  between  signals  of  different  wave 
length  (or  frequency).* 

Crystal  rectified  D,  obviously,  can  be  replaced  by  a  two-electrode  or 
three-electrode  vacuum  valve,  or  by  a  battery  of  valves  if  desired.  The 
system  of  Figure  45  is  applicable  to  the  reception  of  damped  or  undamped 
oscillations. 

32.  SIMPLE  REGENERATIVE  CIRCUIT.— The  circuit  shown 
in  Figure  46  has  been  much  used  for  the  reception  of  damped  and  un- 
damped oscillations.  It  is  merely  an  elaboration  of  that  shown  in  Figure 
43a.  A  point  of  departure  from  other  systems  is  the  method  of  connect- 
ing the  terminals  of  the  secondary  coil  L-2  to  the  valve.  One  terminal 
connects  through  the  grid  condenser  C  to  the  grid  G,  and  the  opposite 
terminal  to  the  plate  P.  There  has  been  much  argument  concerning  the 
functioning  of  this  circuit,  but  careful  scrutiny  reveals  that  the  grid  and 
plate  circuits  are  electrostatically  coupled  through  condenser  C-2.  Ad- 
ditional magnetic  coupling  is  afforded  through  coils  L-3  and  L-4,  but  as 
first  used,  the  condenser  (7-2  provided  the  entire  regenerative  coupling. 
The  inductive  coupling  permits  a  greater  amount  of  energy  to  be  sup- 
plied to  the  grid  circuit  by  the  plate  circuit,  and  is  useful  for  giving 


*This,  of  course,  is  true  of  all  regenerative  circuits. 


76 


Vacuum  Tubes  In  Wireless  Communication 


stability  in  the  reception  of  undamped  oscillations  by  the  beat  method 
(to  be  described  more  in  detail  further  on). 

The  diagram  of  Figure  46  also  shows  how  the  plate  circuit  of  a 
vacuum  valve  may  be  fed  with  direct  current  from  a  dynamo.  Ordi- 
narily the  fluctuations  of  current  due  to  the  commutator  would  cause  an 


HO -500V. 
D.C. 


Figure  46 

Showing  how  the  plate  circuit  of  a  vacuum  tube  may  be  energized 
by  a  direct  current  dynamo.  The  strength  of  the  local  current  is  regu- 
lated by  the  potentiometer  P-l,  and  the  fluctuations  of  the  dynamo  cur- 
rent due  to  the  commutator  are  smoothed  out  by  the  condenser  C-4. 
Inductive  regenerative  coupling  is  provided  between  the  coils  L-3  and 
L-4,  and  electrostatic  regenerative  coupling  by  the  telephone  condenser 
C-2.  The  coil  L-4  is  often  termed  the  "tickler"  coil. 


interfering  "hum"  in  the  receiving  telephones,  but  this  is  largely  pre- 
vented by  shunting  to  the  D.  C.  line  a  condenser  of  large  capacity,  (7-4, 
which  has  the  effect  of  smoothing  out  the  current,  that  is  to  say,  when 
the  current  generated  by  the  dynamo  decreases,  the  energy  stored  up 
in  the  condenser  0-4  discharges  through  the  circuit  and  maintains  the 
line  voltage  at  a  nearly  constant  value.  Potentiometer  P-l,  permits  the 
plate  voltage  to  be  adjusted  to  the  requisite  value,  and  condenser  (7-3  acts 
as  a  by-pass  for  the  incoming  radio  frequency  currents. 


PART  V 

COMBINED  REGENERATIVE  AND  CASCADE 
AMPLIFICATION  SYSTEMS 

33.  IN  GENERAL. — We  next  come  to  amplification  circuits  which 
embrace  the  two  systems  of  amplification  previously  described,  namely, 
the  regenerative  amplifier  and  the  cascade  amplifier.    Other  circuits  than 
those  described  herein  can  be  devised  by  the  experimenter,  the  connec- 
tions for  which  will  suggest  themselves  to  one  who  has  studied  previous 
chapters. 

34.  REGENERATIVE  CASCADE  SYSTEMS.— In  the  diagram 
of  Figure  47,  is  shown  a  system  in  which  the  incoming  oscillations  are 
amplified  in  the  first  valve  through  regenerative  coupling,  the  ampli- 
fied radio  frequency  component  of  the  plate  current  being  impressed 
upon  the  grid  and  filament  of  the  second  valve  where  it  is  further  ampli- 
fied by  a  second  regenerative  coupling,  the  amplified  energy  finally  being 
rectified  and  trapped  in  the  grid  condenser  of  the  last  valve. 

The  student  who  has  carefully  studied  the  principle  of  operation  of 
the  apparatus  noted  in  connection  with  previous  diagrams  will  readily 
recognize  the  features  of  Figure  47. 

First,  the  potential  of  the  grid  in  respect  to  the  filament  of  the 
first  valve  is  adjusted  by  the  grid  battery,  5-3,  and  the  potentiometer 
P-3.  Second,  if  the  voltage  of  the  plate  circuit  and  the  temperature  of 
the  filament  are  adjusted  to  operate  upon  the  slope  A',  Bf  of  the  char- 
acteristic curve  (Figure  22),  substantially  equal  increases  and  decreases 
of  the  plate  current  will  be  secured  for  a  given  incoming  radio  fre- 
quency current  and  a  distortionless  radio  frequency  current  obtained  in 
the  plate  circuit  of  the  first  valve. 

Part  of  the  energy  liberated  by  the  plate  circuit  of  the  first  valve  is 
impressed  upon  its  grid  circuit  through  the  radio  frequency  transformer 

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Mt  which,  in  turn,  causes  an  amplified  radio  frequency  current  in  the 
plate  circuit,  P,  P-l,  C-2.  Through  the  radio  frequency  transformer 
31-1,  this  current  is  impressed  upon  the  grid  circuit  of  the  second  valve, 
8-1,  L-3,  (7-3,  LA. 

The  incoming  signal  is  then  repeated  in  the  plate  circuit  L-5,  C-5, 
L-6,  of  the  second  valve  where  it  is  further  amplified  through  the  re- 
generative coupling  M-2.  The  amplified  radio  frequency  currents  are 
finally  rectified  to  charge  the  grid  condenser  (7-4,  which  exerts  a  relay- 
ing action  on  the  plate  current  of  the  second  valve  at  an  audio  frequency. 
Although  the  plate  circuit  of  the  second  valve  is  tuned  by  the  coils  L-5, 
L  6  and  the  condenser  (7-5,  an  untuned  circuit  may  be  employed. 

It  is  not  always  essential  in  a  system  of  this  kind  that  the  plate  cir- 
cuit of  the  first  valve  be  tuned  to  the  incoming  radio  frequency  currents, 
but  for  the  best  signals  it  is  of  prime  importance  that  the  grid  circuits  of 
both  valves  be  thus  tuned.  This  is  accomplished  mainly  by  condensers 
C-l,  and  (7-3,  the  usual  secondary  condensers.  Variation  of  L-2,  L-3,  L-4, 
P-l,  etc.,  is  necessary  to  obtain  a  wide  range  of  frequencies. 

In  practice,  the  coupling  of  transformer  M-l  is  very  close  and  is 
usually  fixed.  Very  careful  adjustment  of  the  couplings  M  or  M-2  is 
necessary,  or  otherwise  the  valves  will  be  set  into  self-oscillation  at  an 
audio  or  radio  frequency. 

Summarizing  the  functioning  of  the  circuit  of  Figure  47 : 

(1)  The   incoming   radio   frequency   oscillations   are   repeated  in  the 

plate  circuit  of  the  first  valve  and  amplified  through  the  re- 
generative coupling  M,  then  impressed  upon  the  grid  of  the 
second  valve  through  M-l,  and  further  amplified  through 
coupling  M-2. 

(2)  This  amplified  current  is  rectified  by  the   valve  action  between 

G  and  F  and  over  the  duration  of  a  wave  train  a  charge  piles 
up  in  the  grid  condenser  C-4. 

(3)  The  charge  in  this  condenser  raises  the  potential  of  the  grid  to 

a  high  negative  value,  reducing  the  plate  current. 

(4)  At  the  termination  of  the  wave  train  this  charge  leaks  out  the 

grid  condenser  and  the  plate  current  returns  to  normal  value. 
This  variation  of  the  plate  or  telephone  current  occurs  at  an 
audio  frequency. 

This  circuit  will  function  with  or  without  the  grid  condenser  (7-4 
of  the  second  valve.  A  battery  with  a  potentiometer  may  be  connected 
in  the  grid  circuit  and  the  potential  of  the  grid  in  respect  to  the  filament 
adjusted  until  an  assymetrical  relaying  effect  is  obtained  in  valve  No.  2. 
Condenser  C  acts  as  a  by-pass  for  the  radio  frequency  currents  around 
the  potentiometer. 

35.  AUDIO  FREQUENCY  REGENERATIVE  AND  CAS- 
CADE SYSTEMS. — By  employing  the  connections  shown  in  Figure  48, 


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the  radio  frequency  component  of  the  plate  circuit  of  the  first  valve  is 
first  amplified  by  the  regenerative  coupling  M  and  then,  through  the  iron 
core  transformer  M-l,  the  audio  frequency  component  of  the  plate  cur- 
rent is  impressed  upon  the  grid  and  plate  of  the  second  valve. 

In  this  diagram,  the  circuit  of  the  first  valve  includes  the  grid  con- 
denser C,  in  which  the  incoming  oscillations  are  trapped  after  rectifica- 
tion. The  iron  core  transformer  M-l  has  the  primary  winding  P-l  and 
the  secondary  winding  $-1  which  is  connected  to  the  grid  or  input  circuit 
of  the  second  valve.  Condenser  0-2  acts  as  a  by-pass  for  the  radio  fre- 
quency currents  of  the  plate  circuit  around  the  audio  frequency  in- 
ductance P-l.  The  audio  frequency  current  impressed  upon  the  grid 
circuit  of  the  second  valve  is  increased  in  amplitude  by  tuning  the  grid 
circuit  through  the  shunt  condenser  0-3.  The  inductance  of  P-l  and  $-1 
may  be  several  henries.  A  step-up  ratio  of  turns  is  usually  employed. 

The  potential  of  the  grid  circuit  of  the  second  valve  is  adjusted 
to  secure  the  maximum  signals  through  potentiometer  P-l  and  battery 
5-3. 

In  practice  as  many  as  six  valves  have  been  connected  in  cascade 
with  the  connections  of  either  Figure  47  or  48.  Generally,  however,  three 
valves  will  give  sufficient  amplification  for  all  commercial  requirements. 
An  audio  frequency  regenerative  coupling  might  be  employed  with  the 
second  valve  of  Figure  48. 


PART  VI 

THE    VACUUM    TUBE    AS    A    DETECTOR    OF 

CONTINUOUS  WAVES.  AUDIO  FREQUENCY 

TUNING  CIRCUITS.  SPECIAL  CIRCUITS 

FOR  THE  VACUUM  TUBE 

36.  RECEPTION  OF  CONTINUOUS  OSCILLATIONS.— The 

problems  encountered  in  the  reception  of  continuous  waves  in  radio 
telegraphy  have  been  reviewed  briefly  in  the  introduction.  The  vacuum 
tube  can  be  connected  in  many  ways  for  the  reception  of  continuous 
waves  as  will  be  shown  in  the  circuits  following. 

By  way  of  preliminary  explanation,  we  may  assume  that  a  simple 
crystal  rectifier  is  connected  in  the  circuit  of  the  usual  receiving  trans- 
former, and  that  the  complete  receiving  system  is  tuned  to  a  continuous 
wave  transmitter;  as  has  already  been  shown,  a  rectified  current  of  con- 
tinuous amplitude  or  uniform  intensity  would  flow  through  the  tele- 
phone. Because  the  frequency  of  these  rectified  or  direct-current  pulses 
is  above  audibility,  no  sound  will  be  obtained  from  the  telephone  dia- 
phragm except  at  the  opening  and  closing  of  the  circuit.  Let,  however, 
a  mechanical  interrupter,  such  as  the  well-known  "tikker"  or  "slipping 
contact"  detector,  be  connected  in  some  circuit  of  the  receiving  system, 
and  be  adjusted  to  interrupt  the  incoming  signals  from  600  to  900  times 
per  second ;  then  the  telephone  will  be  energized  periodically  at  an  audio 
frequency. 

These  interruptions  (by  the  tikker)  do  not  take  place  in  synchronism 
with  the  incoming  oscillations,  that  is,  energy  is  discharged  into  the  tele- 
phones at  different  portions  on  the  cycles  of  the  incoming  oscillations, 
and  as  a  consequence  a  somewhat  irregular  deflection  of  the  telephone 
diaphragm  is  obtained.  In  other  words,  the  note  produced  in  the  tele- 
phone lacks  the  musical  characteristic  so  desirable  for  working  through 
atmospheric  electricity.  This  defect  is  partially  obviated  by  the  " slip- 

82 


Vacuum  Tubes  In  Wireless  Communication  83 

ping"  contact  detector,  which  impulses  the  telephone  diaphragm  at  a 
slightly  more  uniform  rate. 

The  solution  of  the  problem  is  largely  attained  by  a  form  of  the 
"tikker"  known  as  the  tone-wheel,  which  converts  the  incoming  oscilla- 
tions into  a  practically  uniform  current  of  audio  frequency.  The  signal 
is  therefore  heard  as  a  musical  note  in  the  telephone.  This  desired 
effect  is  obtained  by  driving  the  "tikker"  or  circuit  interrupter  at  such 
speed  that  the  receiving  circuits  are  interrupted  at  a  rate  slightly  off 
synchronism  with  the  incoming  currents,  thus  producing  in  the  tele- 
phone receiver  a  note  having  a  frequency  corresponding  to  the  difference 
between  the  actual  speed  and  the  synchronous  speed. 

To  illustrate  its  action:  If  the  frequency  of  the  incoming  oscilla- 
tions at  any  receiving  station  is  50,000  cycles  per  second,  and  the  tikker 
interrupts  the  circuit  50,000  times  per  second  (in  exact  synchronism  or 
at  the  peak  of  every  other  alternation),  then  either  the  positive  or  nega- 
tive half  of  the  incoming  oscillations  will  be  suppressed  and  the  tele- 
phone will  be  traversed  by  uni-directional  impulses  of  radio  frequency. 
If,  however,  the  "tikker"  interrupts  the  circuit  at  a  rate  different  than 
the  fundamental  frequency,  say,  49,500  times  per  second,  the  wave  form 
is  interrupted  at  points  on  and  off  the  successive  peaks,  which  results  in 
the  production  of  an  audio  frequency  current,  the  frequency  of  which 
is  the  difference  between  the  frequency  of  the  incoming  signals  and  the 
interruptions  of  the  tikker.  In  the  problem  cited,  the  telephone  dia- 
phragm will  be  deflected  500  times  per  second. 

Although  the  Goldschmidt  tone-wheel  is  practicable  for  radio  re- 
ception at  low  frequencies  from  25,000  to  60,000  per  second,  it  is  not 
so  at  the  higher  frequencies  such  as  500,000  cycles  per  second  corres- 
ponding to  the  wave  length  of  600  meters.  The  mechanical  problems  in- 
volved in  the  construction  of  a  device  to  interrupt  the  circuit  one-half 
million  times  per  second  are  too  obvious  for  discussion. 

Another  system  for  obtaining  audible  response  from  radio  frequency 
currents  is  known  as  the  beat  receiver.  A  musical  note  is  secured  in  the- 
telephone  in  this  system,  the  pitch  of  which  may  be  varied  within  the 
limits  of  audibility. 

The  underlying  principle  of  operation  follows:  Two  radio  fre- 
quency currents  of  slightly  different  frequency  superposed  upon  the 
same  circuit,  interact  and  produce  a  third  current  termed  a  "beat"  cur- 
rent.* By  proper  selection  of  the  interacting  frequencies  the  frequencies 
of  the  beats  may  range  from  200  to  1,000  cycles  per  second. 


*See  page  5. 


84 


Vacuum  Tubes  In  Wireless  Communication 


For  example,  if  the  antenna  circuit  of  a  receiving  system  is  adjusted 
to  a  transmitter  whose  frequency  of  oscillation  is  50,000  cycles  per  second, 
and  there  is  induced  in  some  part  of  the  receiving  system  a  locally  gen- 
erated radio  frequency  current  of  49,500  cycles  per  second,  beat  currents 
of  a  frequency  of  500  per  second  will  result.  The  beat  currents  may 
then  be  rectified  to  energize  a  magnetic  head  telephone  in  the  usual 
manner. 

37.  THE  THEORY  OF  THE  BEAT  RECEIVER.— The  gen- 
eral theory  of  the  beat  receiver  may  be  explained  by  the  curves,  Figures 
49  to  52  inclusive.  We  have  stated  in  the  previous  paragraph,  that  the 


X- 


Figure  49 

Showing  the  current  curve  resulting  from  the  addition  of  two  cur- 
rents of  the  same  frequency  but  of  different  amplitude  applied  to  the 
same  circuit. 


requisite  current  of  audio  frequency  for  operation  of  the  head  telephone 
receiver  in  the  beat  system  of  continuous  wave  reception,  is  obtained 
from  the  interaction  of  two  radio  frequency  currents  of  slightly  different 
frequency.  Just  how  this  audio  frequency  current  is  obtained,  can  best 


Vacuum  Tubes  In  Wireless  Communication 


85 


be  understood  by  comparing  the  action  of  two  currents  of  the  same  fre- 
quency which  reach  their  maximum  and  minimum  amplitudes  simul- 
taneously or  at  different  moments ;  that  is,  two  currents  of  the  same  fre- 
quency applied  to  the  same  circuit  identical  in  phase,  or  out  of  phase. 

Let  the  diagram  of  Figure  49  represent  two  currents  in  exact  phase 
in  a  given  circuit,  i.e-,  currents  that  reach  their  maximum  and  minimum 
amplitudes  simultaneously,  but  possess  different  amplitudes.  Curve  X-l 
has  a  maximum  value  of  10  milliamperes,  and  curve  X-2,  15  milliamperes. 
The  resultant  current  will  have  the  maximum  amplitude  shown  by  the 


12 


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Figure  50 

The  curve  resulting  from  two  currents  of  similar  frequency  slightly 
out  of  phase. 


third  curve  X-3,  25  milliamperes.    In  other  words,  the  resulting  current 
is  obtained  simply  by  adding  the  amplitudes  of  the  two  currents. 

In  Figure  50,  the  two  currents  are  out  of  phase  a  few  degrees, 
that  is,  they  do  not  .reach  their  maximum  amplitudes  simultaneously ; 
therefore,  the  amplitude  of  the  resultant  current  will  be  much  less  than 
that  of  Figure  49,  for  reasons  now  to  be  explained :  The  maximum  am- 
plitude of  the  resulting  current  in  this  case  can  be  found  in  the  same 
way,  by  simply  adding  the  values  of  the  amplitude  of  the  currents  at 
any  given  moment  during  the  cycle.  It  should  be  noted,  however,  that 


86  Vacuum  Tubes  In  Wireless  Communication 

at  certain  portions  of  the  cycle  these  two  currents  oppose.  At  P,  Figure 
50,  the  value  of  current  X-l  is  6  milliamperes  and  of  X-2,  13  milli- 
amperes. The  amplitude  of  the  resultant  current  X-3  is  6  +  13,  or  19 
milliamperes.  Again  at  Point  P-l,  the  value  of  current  X-l  is  +  8  milli- 
amperes and  of  current  X-2  —  8  milliamperes.  The  amplitude  of  the 
resultant  current,  therefore,  is  zero,  as  shown  at  point  A  on  the  horizontal 
axis  B,  C. 


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Figure  51 

The  resultant  curve  of  two  currents  180°  out  of  phase  but  of  differ- 
ent amplitude. 


In  the  diagram  of  Figure  51,  two  currents  differ  in  phase  relation 
by  180°  and  therefore  they  oppose  at  all  points  during  the  cycle,  that  is, 
positive  and  negative  maximums  are  reached  simultaneously.  It  is  self- 
evident  that  if  they  possessed  equal  amplitudes  the  amplitude  of  the 
resultant  current  would  be  zero,  but  since  X-l  represents  a  value  of,  let 
us  say,  10  milliamperes  and  X-2  a  value  of  15  milliamperes,  the  maxi- 
mum amplitude  of  the  resultant  current  is  5  milliamperes  as  shown  by 
the  curve  X-3. 

(a)  Phenomenon  of  Beats.  Up  to  this  point,  we  have  dealt  with 
currents  of  similar  frequency,  but  of  different  phase  relation.  When  two 
currents  of  different  frequency  are  applied  to  a  given  circuit,  they  will 
successively  add  up  or  subtract  their  amplitudes,  i.e.,  their  phase  relation 
changes  progressively  from  0°  to  180°  and  back  to  0° ;  that  is,  at  certain 
moments  (during  a  second)  the  two  currents  flow  in  phase  adding  up 


Vacuum  Tubes  In  Wireless  Communication 


87 


their  maximum  values,  while  at  other  moments  they  are  180°  out  of  phase 
and  the  amplitude  of  the  resultant  current  is  zero.  At  all  intermediate 
points,  the  two  currents  either  add  up  or  substract. 

The  interaction  of  the  two  currents  causes  a  periodic  variation  of 
the  amplitude  of  the  resultant  current  which  occurs  at  a  frequency  dif- 
ferent from  either  of  the  applied  frequencies.  This  resultant  current 
is  called  a  "beat"  current,  and  it  can  be  shown  that  the  frequency  of 
the  beat  current  is  equal  to  the  numerical  difference  of  the  two  applied 
frequencies.  Thus,  if  a  current  of  50,000  cycles  per  second  interacts 
with  another  current  of  49,500  cycles  per  second,  the  frequency  of  the 
leat  currents  will  be  50,000  —  49,500  or  500  per  second. 


INCOMING 
OSCILLATIONS 


wvwwvwv 


OSCILLATIONS 


Figure  52 

Graphs  showing  the  beat  currents  resulting  from 
the  interaction  of  two  currents  of  different  frequency. 

It  follows  that  if  a  current  of  100,000  cycles  per  second  interacts 
with  another  of  101,000  cycles  per  second,  the  frequency  of  the  "beats" 
will  be  1,000  per  second,  and  so  on.  The  production  of  beats  by  the  in- 
teraction of  two  currents  can  best  be  explained  by  the  diagram  of  Figure 
52,  wherein  the  line  0-1  represents  a  current  of  250,000  cycles  per 
second,  and  the  line  0-2  a  superposed  current  of  200,000  cycles  per 
second.  The  frequency  of  the  beat  current  shown  on  line  0-3  in  this 
case  is  50,000  cycles  per  second,  which  is,  of  course,  above  the  limits  of 
audibility  and  will  produce  no  sound  in  the  telephone  received.  (A  dif- 
ference of  frequency  so  great  as  this  had  to  be  selected  in  order  that 
several  cycles  of  the  resultant  beat  current  could  be  plotted  to  scale.) 

It  is  now  clear  that  by  proper  selection  of  the  frequencies  of  the 
interacting  currents  beats  of  any  desired  audio  frequency  may  be  secured. 

We  may  review  the  foregoing  facts  as  follows : 

(1)  If  two  currents  of  the  same  frequency  and  like  phase  flow  in  a 
circuit,  the  amplitude  of  the  resultant  current  will  be  the  sum 
of  the  amplitudes  of  the  individual  currents. 


88 


Vacuum  Tubes  In  Wireless  Communication 


(2)  It  two  currents  of  dissimilar  phase  but  of  similar  frequency  flow 

in  a  circuit,  the  amplitude  of  the  resulting  current  is  found 
by  adding  their  amplitudes  at  any  particular  moment. 

(3)  If  the  currents  dift'er  in  phase  by  180°  the  amplitude  of  the  re- 

sultant current  will  be  zero  if  the  two  currents  are  of 
identical  amplitude,  or  if  of  different  amplitude  the  resultant 
current  will  be  the  difference  of  the  amplitude  of  the  two 
currents. 

(4)  If   two   currents   of   unequal   frequency   are   applied   to   a   n^iven 

circuit,  they  will  add  or  subtract  their  amplitudes  periodically 
resulting  in  the  production  of  a  beat  current  the  frequency 
of  which  is  the  numerical  difference  of  the  two  applied 
frequencies. 

We  shall  presently  see  how  this  principle  is  applied  to  the  recep- 
tion of  undamped  or  continuous  waves  in  radio  telegraphy. 

38.  THE  SIMPLE  HETERODYNE.—  It  has  been  already  ex- 
plained that  if  a  receiving  antenna  and  associated  receiver  circuits  con- 
taining a  simple  rectifier  are  tuned  to  a  continuous  wave  transmitter, 
the  receiving  telephone  will  be  traversed  by  a  uni-directional  current  as 


Figure  53 

Showing  the  rectified  current  pulses  in  the  local 
telephone  circuit  of  a  receiving  system  containing  a 
simple  rectifier. 


A 


0-2 


Figure  54 

Showing  how  the  amplitude  of  direct  current 
pulses  (above  audition)  can  be  varied  at  an  audio 
frequency. 


shown  in  graph  0-1,  Figure  53.  This  represents  the  rectified  current 
of  an  incoming  group  of  oscillations.  The  individual  pulses  of  the 
graph  occur  at  a  radio  frequency  and  therefore,  the  telephone  emits  no 
sound.  But  if  by  local  means  we  cause  the  amplitude  of  these  pulses 
to  fluctuate  periodically  at  an  audio  frequency  as  shown  in  graph  0-1, 


Vacuum  Tubes  In  Wireless  Communication 


89 


Figure  54,  sound  will  be  produced  because  the  telephone  diaphragm  will 
be  deflected  at  rates  varying  as  the  periodic  fluctuations  of  the  rectified 
current. 

The  telephone  current  will  be  approximately  that  of  the  graph  0-2, 
in  which  successive  curves  A,  B,  G  represent  the  average  effect  of  the 
individual  groups  of  pulses  in  the  graph  0-1.  This  is  what  occurs  in 
the  circuits  of  the  simple  heterodyne  receiver  shown  in  Figure  55. 

As  in  the  usual  receiving  circuit  employing  a  crystal  or  solid  recti- 
fier, the  primary  winding  is  represented  by  the  coil  Zr-1,  the  secondary 
winding  by  the  coil  L-2,  the  crystal  detector  by  D,  the  shunt  secondary 
condenser  by  (7-2,  the  telephone  condenser  by  0-1,  and  the  head  tele- 
phone by  P.  A  radio  frequency  alternator  of  variable  frequency  A-l,  is 
inductively  coupled  through  L-3  to  the  antenna  coil  LA. 

The  operation  of  this  system  in  brief  is  as  follows :  Assume  that  the 
antenna  circuit  L-l,  LA,  A,  E,  is  tuned  to  8,000  meters,  the  oscillation 

frequency  of  this  circuit  is        '       '        or,  37,500  cycles  per  second.    The 

8,000 

frequency  of  the  current  induced  in  the  secondary  circuit  L-2,  0-2  and 
impressed  across  the  oscillation  detector  D,  is  also  37,500  cycles  per 
second,  and,  for  reasons  already  explained,  the  telephone  diaphragm 
emits  no  sound.  But  if  the  frequency  of  the  alternator  is  36,500  cycles 


A-i 


v C-3 


Figure  55 

The  circuits  of  a  simple  beat  receiver.  Local  oscillations  are  gen- 
erated by  the  radio  frequency  alternator,  vacuum  tube  or  arc  generator 
A-l.  The  resulting  beat  currents  are  detected  in  the  circuit  L-2,  C-2,  D, 
C-l,  P. 


90 


Vacuum  Tubes  In  Wireless  Communication 


per  second,  the  amplitude  of  the  oscillations  in  the  entire  system  will  be 
varied  from  a  maximum  to  a  minimum  at  a  rate  due  to  the  difference  of 
37,500  and  36,500  or  1,000  per  second,  i.e.,  the  frequency  of  the  "beat" 
currents  will  be  1,000  per  second.  These  beats  in  turn  are  rectified  by 
the  detector  D  and  1,000  pulses  of  direct  current  per  second  energize  the 
telephone  P. 

The  same  beat  frequency  will  be  obtained  if  the  alternator  A-l  is 
adjusted  for  38,500  cycles  per  second;  that  is,  the  frequency  of  the 
"beats"  in  every  case  equals  the  difference  of  the  two  applied  frequencies. 

By  changing  the  frequency  of  the  alternator  A-\J  the  receiving  opera- 
tor can  change  the  pitch  of  the  beat  note  from,  say,  200  cycles  to  1,000 
cycles  per  second  or  beyond  the  limits  of  audibility.  Not  only  can  the 
note  be  varied  according  to  the  desire  of  the  operator,  but  it  can  be 
shown  that  the  efficiency  of  the  receiver  is  thus  increased,  i.e.,  amplifica- 
tion of  the  incoming  signal  is  secured. 


O-i 


o-z 


INCOMING 
OSCILATIONS 


BEAT  CURRENT 


RECTIFIED 
BEAT  CURRENT 


O-4 


AAAflfJA.aAA 


0-5 


PERIODIC 
TELEPHONE  CURRENT 


Figure  56 
Curves  showing  the  functioning  of  the  beat  receiver. 


(a)  Curves  of  the  Beat  Receiver  in  Radio. —  The  complete  pro- 
cesses involved  in  the  detection  of  continuous  oscillations  by  this  method 
(Figure  55)  are  shown  graphically  in  the  series  of  curves.  Figure  56; 


Vacuum  Tubes  In  Wireless  Communication  91 

where  0-1  represents  the  incoming  oscillations ;  0-2,  the  locally  generated 
current ;  0-3,  the  resulting  beat  current ;  0-4  the  rectified  beat  current ; 
and  0-5,  the  approximate  telephone  current.  It  is  to  be  noted  that  the 
amplitude  of  the  beat  current  exceeds  the  amplitude  of  either  the  locally 
generated  current  or  that  of  the  incoming  signal. 

It  would  be  well  for  the  student  here,  to  bear  in  mind  that  the  radio 
frequency  alternator  A-l  can  be  replaced  by: 

(1)  An  arc  generator  such  as  the  Poulsen  type. 

(2)  A  vacuum  tube  oscillator. 

The  complete  circuit  for  (2)  is  shown  and  described  in  paragraph  41. 

39.  SELECTIVITY  BY  THE  HETERODYNE.— In  addition  to 
the  selectivity  afforded  by  the  usual  radio  frequency  tuning  of  the  re- 
ceiver circuits,  an  additional  discrimination  between  different  stations 
operating  near  the  same  wave  is  secured  by  reason  of  the  beat  phe- 
nomenon. 

If  the  frequency  of  the  incoming  oscillations  from  the  station  desired 
is,  for  example,  37,500  cycles,  per  second  (A  =  8,000  meters)  and  the 
frequency  of  the  local  oscillations  36,500  cycles  (A  =  8,219  meters)  the 
beat  pitch  will  be  1,000  per  second.  Suppose,  however,  that  another  sta- 
tion sends  at  a  wave  length  corresponding  to  a  frequency  of  37,000  cycles 
per  second  (A  =  8,108  meters),  then  a  beat  note  of  500  per  second  will 
be  obtained,  and  due  to  the  differences  of  tone,  the  operator  can  concen- 
trate on  the  particular  signal  he  desires  to  receive. 

Then  if  several  undamped  wave  transmitters  operate  simultaneously 
at  such  wave  lengths  as  to  produce  a  beat  note  in  the  receiving  apparatus 
when  its  locally  generated  current  is  adjusted  to  a  certain  frequency,  the 
operator  in  many  cases  can  change  the  pitch  of  the  beat  note  of  the 
station  he  desires  to  receive,  to  a  pitch  that  will  permit  him  to  concen- 
trate on  that  signal  to  the  exclusion  of  others. 

Also,  if  the  frequency  of  the  local  current  is  maintained  at  a  definite 
value,  signals  will  be  heard  only  from  such  stations  as  will  produce  a 
beat  pitch  between  the  values  of,  say,  16  to  10,000  per  second.  It  must 
be  remembered,  however,  that  the  best  response  is  generally  obtained 
when  the  beat  pitch  is  somewhere  near  the  value  of  1,000  per  second. 

It  is  easily  seen  from  the  foregoing,  that  if  the  frequency  of  an  inter- 
fering signal  is  sufficient  to  cause  a  beat  current  of  a  frequency  above 
or  below  the  limits  of  audibility,  it  will  not  be  heard  in  the  telephone 
even  if  the  receiving  station  is  near  enough  to  the  transmitter  to  be 
energized  by  forced  oscillations, 


92 


Vacuum  Tubes  In  Wireless  Communication 


As  will  be  shown  in  paragraph  43  an  audio  frequency  tuning  cir- 
cuit can  be  connected  to  the  plate  circuit  of  the  vacuum  tube  detector, 
requiring  the  receiving  operator  to  tune  to  the  beat  frequency  as  well  as 
to  the  radio  frequency  oscillations. 

In  summary,  there  are  three  modes  of  obtaining  selectivity  in  a 
beat  receiver: 

(1)  by  radio  frequency  tuning; 

(2)  by  change  of  the  beat  note; 

(3)  by  audio  frequency  tuning. 

The  student  will  comprehend  this  more  clearly  from  the  instruc- 
tions to  follow. 


DETECTOR  CIRCUIT 


LOCAL  GENERATOR 


Figure  57 
The  vacuum  tube  as  a  detector  in  the  beat  receiver. 


40.  HETERODYNE  WITH  THE  VACUUM  VALVE  AS  A 
DETECTOR. —  "We  next  consider  the  use  of  the  three  electrode  vacuum 
tube  as  a  detector  in  the  heterodyne  system  in  place  of  the  carborundum 
rectifier.  In  the  diagram  of  Figure  57,  the  crystal  rectifier  of  Figure  55 
has  been  replaced  by  a  vacuum  tube  F,  G,  P}  adjusted  for  assymetri&al 
relaying,  i.e.,  the  potential  of  the  grid  to  filament  is  controlled  by  a  grid 
battery  B-3,  to  secure  the  best  relaying  effect. 

If  the  alternator  A-l,  adjusted  to  a  frequency  slightly  off  the  signal 
frequency  is  coupled  to  the  antenna  at  coils  L-3  and  I/-4,  an  E.M.F. 
at  a  radio  frequency  will  be  induced  across  the  terminals  of  LA.  Cur- 
rents of  similar  frequency  will  flow  in  the  complete  antenna  system. 
Currents  of  the  same  frequency  will  be  induced  in  the  secondary  or 
detector  circuit  L-2,  C-l,  and  the  plate  current  will  rise  and  fall  at  a 


Vacuum  Tubes  In  Wireless  Communication 


93 


radio  frequency  as  shown  by  the  graph,  Figure  58.  Since  the  variations 
of  the  plate  current  occur  at  a  radio  frequency,  and  have  constant  ampli- 
tude, no  response  is  obtained  in  the  head  telephone  P,  but  this  current 
flows  in  the  plate  or  output  circuit  so  long  as  the  alternator  A-I  is  in 
operation. 

Assume,  however,  that  a  distant  transmitter  induces  continuous  oscil- 
lations in  the  receiving  aerial  at  a  frequency  differing  from  the 
local  frequency  by  500  to  1,000  cycles;  these  oscillations  will  interact 
Avith  the  locally  generated  oscillations  producing  beats  which  have  ampli- 


PLATt    CURRENT 


Figure  58 

Showing  the  repeated  oscillations  (in  the  plate 
circuit)  when  the  valve  is  operated  at  a  certain  point 
on  the  characteristic  curve.  Undamped  oscillations  are 
assumed  to  be  impressed  upon  the  grid  circuit. 


tude  greater  than  either  the  signalling  frequency  or  the  local  frequency. 
That  is,  as  the  two  currents  add  up  or  subtract,  the  potential  of  the 
grid  will  be  raised  and  lowered  above  and  below  the  potential  supplied 
by  the  incoming  signal.  Then,  if  the  grid  potential  is  adjusted  so  that 
a  rectified  current  flows  in  the  plate  circuit,  the  telephone  will  respond 
to  an  average  effect  of  each  group  of  beat  currents.  In  this  way  the 
plate  current  varies  at  the  frequency  of  the  beats  and  the  telephone 
diaphragm  is  deflected  accordingly. 

Diagrammatically,  the  actions  of  the  circuit  in  Figure  57  are  shown 
in  Figure  59,  it  being  assumed  that  the  grid  potential  of  the  detecting 
tube  is  adjusted  so  that  a  rectified  current  flows  in  the  plate  circuit. 
Curve  0-1  represents  the  frequency  of  the  incoming  signal;  curve  0-2, 
the  locally  generated  current ;  curve  0-3,  the  beat  current ;  curve  0-4, 
the  modulated  plate  current,  and  curve  0-5,  the  approximate  telephone 
current. 

Further  amplification  of  the  beat  current  can  be  secured  by  pro- 
viding the  system  of  Figure  57  with  a  regenerative  coupling  (audio  or 
radio  frequency),  but  the  coupling  must  not  be  too  close  or  the  valve 
will  be  set  into  self-oscillation — a  state  of  affairs  not  desired  with  this 
particular  set  of  connections. 


94 


Vacuum  Tubes  In  Wireless  Communication 


41.  THE  VACUUM  VALVE  AS  A  GENERATOR  OF  RADIO 
FREQUENCY  OSCILLATIONS.—  A  vacuum  tube  connected  as  in 
Figure  60  may  be  employed  to  generate  the  local  radio  frequency  cur- 
rents for  beat  reception.  The  grid  and  plate  circuits  are  coupled 
through  coils  L-2  and  L-3.  The  grid  circuit  is  tuned  to  a  given  fre- 
quency of  oscillation  by  the  condenser  (7-1  and  the  plate  circuit  by  con- 
denser (7-2. 


o-i 


INCOMING 
SIGNALS 


0-5 


Figure  59 

Illustrating  the  actions  of  the  three-electrode  tube  for  the  detection  of 
beat  currents. 

The  conditions  present  in  the  plate  circuit  are  substantially  those 
of  the  circuit,  Figure  61,  where  an  oscillation  circuit  including  coil  L 
and  condenser  (7  is  shunted  by  the  battery  B,  the  circuit  being  impulsed 
by  opening  and  closing  the  switch  8. 

When  the  switch  8  is  closed,  the  energy  supplied  by  the  battery 
is  stored  temporarily  in  coil  L  and  condenser  (7;  but  when  switch  8  is 
opened  this  stored  up  energy  is  released  and  the  circuit  L,  C,  oscillates 
at  a  radio  frequency  determined  by  the  product  of  the  capacity  and  in- 


Vacuum  Tubes  In  Wireless  Communication 


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ductance  of  the  circuit.  Now,  switch  8  can  be  considered  to  be  replaced 
in  Figure  60  by  the  valve  grid  G,  which  varies  the  current  of  B-2  by 
change  of  its  potential. 

It  has  been  shown  that  if  the  grid  is  charged  to  a  high  negative 
potential,  the  circuit  of  B-2  is  completely  opened  (but  this  does  not  occur 
in  practical  operation)  and  consequently,  it  is  easily  seen  that  any  vari- 


Figure  60 

The  circuits  of  the  vacuum  tube  for  generating  radio  frequency 
currents.* 


Figure   61 

An  oscillation  circuit  set  into  excitation  by  an 
external  E.  M.  F. 

able  E.M.F.  applied  to  the  grid  circuit,  will  vary  the  strength  of  the 
plate  current,  which  will  set  circuit  L-3,  (7-2,  LA  into  oscillation  at  a 
radio  frequency. 

Through  the  coupling  L-2,  L-3,  the  grid  circuit  L-25  0-1,  will  be 
set  into  oscillation,  and  it  will  vary  the  plate  current  through  change 
of  the  grid  potential.  This  will  occur  in  synchronism  with  the  radio 
frequency  current.  In  this  way  the  complete  system  oscillates  at  a  radio 
frequency  which  may  be  varied  over  a  wide  range  of  frequencies  by 


*The  grid  battery  B-4  is  not  essential  in  this  circuit. 


96 


Vacuum  Tubes  In  Wireless  Communication 


change  of  capacity  of  condensers  (7-1  and  (7-2,  or  by  variation  of  induc- 
tances L-2,  L-3  and  LA.  It  is  found,  in  practice,  that  the  condenser  C-2 
and  the  coil  LA  can  be  eliminated,  sufficient  energy  being  liberated  in  the 
plate  circuit  to  keep  the  valve  circuits  in  self -oscillation  through  the  coup- 
ling coils  L-2,  L-3. 

It  should  now  be  clear  that  in  order  to  set  the  valves  into  self- 
oscillation,  it  is  necessary  to  change  the  potential  of  either  the  grid  or 
plate  circuits,  and  to  provide  static  or  magnetic  coupling  between  the 
circuits  in  order  that  some  of  the  energy  released  by  the  plate  circuit 
may  be  fed  back  into  the  system.  A  switch  may  be  connected  in  the 
circuit  of  the  battery  BA  to  set  the  system  into  oscillation.  It  is  fre- 
quently found  that  sudden  variations  of  the  capacity  of  condenser  (7-1, 
or  (7-2,  will  vary  the  potential  sufficiently  for  the  purpose. 

Connected  as  in  Figure  60  the  valve  may  be  employed  to  generate 
the  local  oscillations  for  the  beat  system  shown  in  Figure  57.  The  receiv- 
ing antenna  is  coupled  to  either  coil  L-2  or  L-3. 


BEAT  DETECTOR 


LOCAL  GENERATOR 


Figure  62 

The  vacuum  tube  connected  up  for  generation  of  radio  frequency 
oscillations  for  the  production  of  beat  currents.  This  system  is  some- 
times called  the  external  heterodyne. 


A  simpler  valve  circuit  for  the  generation  of  sustained  oscillations 
is  shown  in  Figure  62,  where  the  grid  and  plate  circuits  are  magnetically 
coupled  through  the  coil  L ;  additional  regenerative  coupling  is  provided 
by  condenser  C.  By  proper  selection  of  the  values  of  L  and  (7,  the  valve 


Vacuum  Tubes  In  Wireless  Communication 


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circuits  will  oscillate  vigorously  at  any  desired  frequency.  These  oscilla- 
tions interacting  with  the  oscillations1  of  the  incoming  signal  produce 
beats  in  the  detector  circuits  which  in  turn  are  detected  by  the  valve 
acting  in  either  of  the  ways  previously  outlined. 

42.  THE  REGENERATIVE  BEAT  RECEIVER.— In  the  hetero- 
dyne or  beat  receiver  circuits  heretofore  shown,  the  local  frequency  is 
generated  by  an  external  oscillator,  and  the  beat  currents  are  detected 
in  a  coupled  detector  circuit.  Such  a  receiver  may  be  termed  the  external 
heterodyne. 

We  now  come  to  a  regenerative  receiver  in  which  the  combined  func- 
tions of  regenerative  amplification,  the  generation  of  local  oscillations,  the 
production  of  beats  and  detection  occur  in  the  circuits  of  a  single  valve 
bulb.  Such  a  system  is  generally  known  as  the  regenerative  beat  receiver } 
but  is  also  termed  the  self -heterodyne  or  internal  heterodyne. 


B-2 


Figure  63 

The  vacuum  tube  as  a  self-heterodyne  or  as  a  detector  of  con- 
tinuous waves.     (Armstrong's  Regenerative  System). 


All  of  the  several  regenerative  circuits  already  shown  can  be  set 
into  self  oscillation  at  radio  frequencies  provided  the  coupling  of  the 
plate  and  grid  circuits  is  sufficiently  close.  For  example,  the  circuit 
shown  in  Figure  38  and  reproduced  in  Figure  63,  indicates  a  system 
which  will  generate  undamped  or  continuous  oscillations  of  any  desired 
frequency,  but  which  simultaneously  will  act  as  a  detector  of  incoming 
radio  signals.  This  is  one  of  Armstrong's  original  regenerative  circuits. 

The  plate  circuit  in  Figure  63  is  tuned  to  the  incoming  signal  by 
X-3,  C-3,  P  (0-2  may  be  fixed),  and  the  grid  circuit  by  L-2,  S,  C-l. 
Usually  a  fixed  capacity  can  be  found  for  condenser,  C,  approximately 
.00003  microfarad. 


98  Vacuum  Tubes  In  Wireless  Communication 

(a)  How  the  Valve  is  Set  into  Self-Oscillation.  We  have  already 
explained  in  a  general  way  the  process  by  which  the  valve  may  be  made 
to   generate   radio   frequency   currents.      Imagine   any   disturbance   to 
occur  in  the  plate  circuit  such  as  a  sudden  variation  of  the  plate  volt- 
age— the  resulting  change  of  current  through  P  causes  it  to  act  induc- 
tively on  8,  setting  the  grid  circuit  L-2,  C-l,  8,  into  oscillation.  The  grid 
then  acts  to  vary  the  plate  current  at  a  radio  frequency.     Part  of  the 
plate  circuit's  energy  is  fed  back  to  the  grid  circuit  to  keep  the  entire 
system  in  self  oscillation.     This  state  of  oscillation  is  only  possible  by 
reason  of  the  fact  that  the  energy  released  in  the  plate  circuit  is  equal 
to,  or  exceeds  that  applied  to  the  grid  circuit,  and  consequently  part  of 
the  energy  liberated  by  the  plate  circuit  can  be  re-transferred  to  the 
grid  circuit  through  the  regenerative  coupling  to  keep  the  system  in 
continuous  oscillation. 

The  frequency  of  the  oscillations  generated  by  the  valve  of  Figure 
63,  will  be  approximately  that  of  the  grid  circuit,  if  the  capacity  of  the 
secondary  condenser  exceeds  the  capacity  of  the  valve  (grid  to  filament), 
but  if  it  is  less  than  the  valve  capacity,  the  inductance  and  capacity  of 
the  plate  circuit  will  be  the  governing  factor. 

(b)  The  Phenomena  of  the  Self-Heterodyne.     The  sequence  of 
events  when  the  valve  of  Figure  63  is  set  into  self  oscillation,  has  been 
explained  graphically  by  Armstrong  as  in  the  curves  of  Figure  64,  where 
graph  0-1  shows  the  gradual  building  up  of  oscillations  in  the  grid  cir- 
cuit by  regenerative  coupling.    These  oscillations  are  rectified  and  charge 
the  grid  condenser  C,  the  charge  in  which  leaks  off  constantly  either 
through  the  valve  or  across  a  special  leak  resistance  (R-l,  Figure  63)  of 
several  hundred  thousand  ohms. 

Now,  the  rectified  current  charges  the  grid  negatively  and,  as  already 
explained,  an  increasing  negative  charge  decreases  the  plate  current  and 
consequently  reduces  the  amplitude  of  the  grid  circuit  oscillations — 
less  energy  being  supplied  through  the  regenerative  coupling  M.  This 
reduction  continues  until  the  amount  of  electricity  supplied  to  the  grid 
condenser  is  equal  to  the  rate  at  which  it  leaks  off. 

The  valve  then  oscillates  steadily  as  shown  by  the  curve  0-2,  which 
represents  the  potential  of  the  grid  to  filament.  The  plate  current  under- 
goes variation  at  the  same  frequency  as  shown  by  0-3.  The  steady  tele- 
phone current  is  shown  by  the  curve  0-4. 

Let  the  valve  circuits  oscillate  steadily  as  shown  in  the  curves  0-2 
and  0-3,  and  let  the  frequency  differ  slightly  from  that  of  the  incoming 
signal;  independently  of  the  local  oscillations  the  incoming  oscillations 
change  the  grid  potential,  are  repeated  in  the  plate  circuit  and  are  re- 


Vacuum  Tubes  In  Wireless  Communication 


99 


enforced  in  the  grid  circuit  through  the  regenerative  coupling  M  (Figure 
63).  Simultaneously  these  amplified  oscillations  interact  with  the  local 
oscillations  producing  throughout  the  system  the  beats  shown  by  line 
0-1  (Figure  65). 


CURRENT  IN 
GRID  CIRCUIT 


0-2 


0-3 


O-4 


TELEPHONE 
CURRENT 


Figure  64 

Showing  the  phenomena  involved  when  the  vacuum  tube  of  figure 
63  is  set  into  self-oscillation. 


These  beats  alternately  increase  or  decrease  the  potential  of  the 
grid  above  and  below  that  steady  state  indicated  at  0-2  in  Figure  64. 
Hence,  the  potential  of  the  grid  to  the  filament  varies  as  in  curve  0-2, 
Figure  65,  and  the  plate  current  fluctuates  at  an  audio  frequency  as  in 
0-3 ;  that  is,  the  amplitude  of  the  repeated  radio  frequency  plate  current 
varies  at  an  audio  frequency,  and  the  approximate  telephone  current  is 
that  shown  in  the  curve  0-4  (Figure  65). 

As  pointed  out  by  Captain  Armstrong,  the  great  advantage  of  this 


100 


Vacuum  Tubes  In  Wireless  Communication 


system  of  reception  lies  in  the  fact  that  the  telephones  are  traversed 
by  an  almost  simple  harmonic  alternating  current.  Hence,  the  telephone 
receivers  may  be  connected  in  a  coupled  audio  frequency  tuning  circuit 
where  selectivity  or  tuning  independent  of  the  signalling  frequency  may 


BEAT 

CURRENT 


o-z 


POTENTIAL  OF  GRID 
TO  FILAMENT 


0-3 


PLATE 

CURRENT 


0-4 


Figure  65 

Illustrating  the  resulting  beat  currents  and  the  final  effect  upon  the 
telephone  current  of  the  vacuum  tube,  connected  as  in  figure  63. 


be  secured ;  that  is,  an  audio  frequency  tuning  circuit  may  be  coupled  to 
the  output  circuit  of  the  valve,  in  which  oscillations  of  different  group 
frequencies  can  be  tuned  in  or  out  as  desired. 

43.  AUDIO  FREQUENCY  TUNING.— In  the  circuit  for  group 
frequency  (audio  frequency)  tuning  shown  in  Figure  66,  the  primary 
and  secondary  circuits  of  the  receiving  transformer  have  been  eliminated ; 
connection  in  practice  being  made  to  terminals  A,  B.  The  group  fre- 
quency tuner  embraces  the  telephone  P,  the  variable  condenser  C,  and 
the  secondary  winding  $'  which  is  coupled  to  the  plate  circuit  of  the 
vacuum  valve  through  winding  P7,  shunted  by  condenser  C-2. 

In  order  to  secure  effective  group  tuning  the  inductance  of  the  coil 


Vacuum  Tubes  In  Wir&Le^s  , 


101 


S  must  be  extremely  large  to  obtain  the  necessary  ratio  of  reactance  of 
the  circuit. 

The  use  of  this  circuit  in  practice  may  be  explained  as  follows: 
Assume  that  during  the  reception  of  undamped  waves,  several  beat  fre- 


Figure  66 
Audio  frequency  tuning  circuit. 


quencies  are  obtained  in  the  plate  circuit  of  the  vacuum  tube;  the  re- 
ceiving operator  may  obtain  response  from  the  signal  desired  by  care- 
fully tuning  the  audio  frequency  circuit  to  a  particular  beat  pitch  to 
the  complete  or  partial  exclusion  of  the  unwanted  signals.  Selectivity 
is  thus  secured  independently  of  the  usual  radio  frequency  tuning. 


VALVE  N21 


VALVE  N«2 


«  ^ 


Figure  67 

Two-step  vacuum  tube  cascade  amplifier.     The  grid  circuit  of  the 
amplifying  tube  is  tuned  to  audio  frequencies. 

A  more  desirable  circuit  for  audio  frequency  tuning  is  shown  in 
Figure  67,  where  a  second  vacuum  valve  with  a  tuned  audio  frequency 
grid  circuit  S,  L-5,  0-4,  is  coupled  to  the  plate  circuit  of  the  first 
valve  through  the  audio  frequency  transformer  M.  By  keeping  the  re- 
sistance of  this  circuit  low,  response  may  be  obtained  in  the  output  cir- 
cuit of  the  second  valve  at  audio  frequencies  with  much  lower  values 


102 


Vacuum  Tubes  In  Wireless  Communication 


of  inductance  in  the  grid  circuit  than  those  employed  in  the  audio  fre- 
quency tuning  circuit  of  Figure  66. 

In  the  diagram,  Figure  67,  S,  L-5,  C-4,  are  of  the  correct  value 
to  permit  tuning  to  frequencies  from  say  200  to  1,000  cycles  per  second. 
By  means  of  battery  5-4,  and  potentiometer  P-l,  the  potential  of  the 
grid  in  respect  to  the  filament  in  the  second  valve  is  adjusted  to  obtain 
the  best  amplification.  The  condenser  0-1  in  the  plate  circuit  of  the 
first  valve  serves  as  a  by-pass  for  the  radio  frequency  component  of  the 
plate  current  around  the  audio  frequency  inductance  P.  Coil  L-3  per- 
mits the  plate  circuit  of  the  first  valve  to  be  tuned  to  the  incoming  sig- 
nals, and  regenerative  coupling  as  usual  is  obtained  at  the  radio  fre- 
quency transformer  M-l. 

In  order  that  full  advantage  may  be  taken  of  an  audio  frequency 
tuning  circuit,  it  is  essential  that  the  frequency  of  the  wave  generator 
remain  constant,  for  it  is  readily  seen  that  if  its  frequency  changes,  the 
beat  note  will  change  at  the  receiver.  This  may  put  the  beat  frequency 
out  of  range  with  the  sharply  tuned  audio-frequency  circuit. 


_L 


T...C-I 


E-i 


Figure  68 
Weagant's  receiver  for  damped  and  undamped  oscillations. 

A  phenomenon  of  this  circuit  which  causes  it  to  operate  at  a  dis- 
advantage is  the  following:  An  impulse  of  static  or  atmospheric  elec- 
tricity will  set  the  audio  frequency  circuit  into  oscillation  by  shock  exci- 
tation converting  the  otherwise  low-pitched  static  signals  into  musical 
sounds. 

In  event  that  static  is  not  severe  and  interference  is  experienced 
from  another  station,  some  advantage  is  derived  in  the  use  of  this  circuit. 

44.  THE   WEAGANT   UNDAMPED    WAVE    RECEIVER.— 

Regenerative  couplings  such  as  previously  shown  are  not  essential  to 
the  production  of  beat  currents  in  the  reception  of  continuous  waves. 
The  system  of  Figure  68,  devised  by  Mr.  Roy  A.  Weagant,  functions 


Vacuum  Tubes  In  Wireless  Communication 


103 


satisfactorily  with  the  simple  set  of  connections  shown.  The  principle 
feature  of  this  system  is  the  "X,"  or  tertiary  circuit,  consisting  of  the 
condenser  (7-3  and  the  inductance  L-3,  which  are  connected  in  series,  and 
in  shunt  to  the  plate  circuit. 

By  carefully  adjusting  the  capacity  of  the  condensers  in  the 
secondary  and  "X"  circuits,  or  change  of  both  the  inductance  and 
capacity,  the  system  will  oscillate  at  radio  frequency  which  may  be 
made  to  differ  slightly  from  the  frequency  of  the  incoming  signal.  Beats 
of  any  desired  frequency  are  thus  formed. 

In  practice,  the  author  has  found  this  system  to  give  practically 
the  same  strength  of  signal  as  any  of  the  regenerative  circuits  previously 
shown.  It  is  applicable  either  to  damped  or  undamped  oscillations.  On 
account  of  its  extreme  simplicity  the  circuit  is  particularly  recom- 
mended. 

45.  MODIFIED  WEAGANT  BEAT  RECEIVER.—The  circuits 
of  Figure  68  may  be  modified  as  in  Figure  69.  It  is  simply  an  exten- 
sion of  the  "X"  circuit,  inductive  regenerative  coupling  being  secured 
through  the  coupling  L-4,  L-5. 


Figure  69 
Modified  Weagant  receiver  for  continuous  and  discontinuous  waves. 


In  Figure  69,  the  aerial  tuning  inductance  is  indicated  at  L-l,  the 
primary  winding  of  the  receiving  tuner  at  L-2,  the  secondary  winding  at 
L-3,  the  secondary  loading  inductance  at  L-4,  and  the  shunt  secondary 
condenser  at  (7-1.  The  usual  grid  condenser  is  indicated  at  (7  and  the 


104  Vacuum  Tubes  In  Wireless  Communication 

tuning  elements  of  the  plate  circuit  at  L-5,  C-2.  LA  and  L-5  are  placed 
in  inductive  relation  for  regenerative  amplification. 

It  is  preferable  in  a  circuit  of  this  kind  that  the  secondary  induct- 
ance LA  be  connected  at  the  upper  end  of  the  coil  L-3  as  indicated 
in  the  drawing,  since  the  valve  functions  best  in  a  secondary  circuit 
giving  the  maximum  possible  voltage  for  a  given  incoming  signal.  The 
point  of  maximum  potential  in  this  case  is  at  the  right-hand  end  of  the 
coil  LA-,  that  is,  the  greatest  E.M.F.  exists  at  the  end  of  LA  furthest 
away  from  the  earth  end  of  L-3. 

In  the  reception  of  undamped  waves  the  operator  adjusts  the  appa- 
ratus first  by  regulating  the  incandescence  of  lamp  filament  and  the 
voltage  of  the  battery  B-2.  He  then  tunes  the  grid  and  plate  circuits  to 
the  requisite  frequency  for  the  production  of  beat  currents.  The  sta- 
bility of  the  circuit  with  some  valves  is  somewhat  enchanced  by  adjust- 
ing the  coupling  between  LA  and  L-5,  but  it  is  not  essential  to  practical 
operation  in  the  majority  of  cases. 

The  inductance  of  LA  and  L-5  may  be  fixed.  A  wide  range  of  fre- 
quencies may  be  secured  by  simultaneous  adjustment  of  the  variable 
condensers  (7-1  and  C-2. 

The  antenna  must,  of  course,  be  tuned  to  the  incoming  signal  by 
inductances  L-l  and  L-2,  or  at  the  variometer  V,  but  in  general,  these 
three  adjustments  cover  substantially  all  operations  necessary  in  prac- 
tice to  tune  the  circuits  to  the  distant  transmitter. 

Inductive  disturbances  from  nearby  60  cycle  alternating  current 
circuits  may  be  eliminated  by  connecting  the  valve  filament  to  earth  at 
point  E-l. 

For  wave  lengths  up  to  10,000  meters,  coils  LA  and  L-5  if  of  the 
single  layer  type  are  from  20  to  24  inches  in  length  and  from  5  to  6 
inches  in  diameter  wound  with  No.  30  or  32  B.  &  S.  wire.  The  author, 
in  fact,  has  secured  very  good  results  with  much  smaller  coils  wound 
with  No.  36  B.  &  S.  wire.  Multi-layered  coils  are  frequently  used  in 
circuits  of  this  kind,  and  of  course  possess  smaller  dimensions  for  a 
given  wave  length  than  any  of  the  other  types  mentioned.  The  receiv- 
ing transformer  L-2,  L-3  for  use  in  connection  with  the  loading  coils 
mentioned  above  may  have  the  usual  dimensions  for  wave  lengths  up 
to  4,000  meters. 

A  variometer  inductance  is  of  considerable  advantage  in  the  beat 
receiver  circuits. 

The  effect  of  turning  the  handle  of  the  variometer  during  beat  re- 
ception from  a  continuous  wave  transmitter  is  of  interest.  Assume  that 
with  the  control  handle  at  the  "half-way"  position  a  beat  note  equal  to 
a  pitch  of  500  is  secured  in  the  head  telephones,  then  by  simply  turning 


/  Vacuum  Tubes  In  Wireless  Communication  105 

the  handle  (of  the  variometer)  thereby  detuning  the  antenna  circuit,  the 
pitch  of  the  beat  note  will  change  from  a  higher  to  a  lower  note  accord- 
ingly as  the  tuning  is  varied. 

As  a  matter  of  scientific  interest,  there  has  been  included  in  dotted 
lines  in  the  diagram  of  Figure  69,  a  so-called  "sensitizing"  circuit,  L-6, 
(7-6,  first  published  by  Dr.  Austin. 

It  has  been  assumed  by  Dr.  Austin  that  the  coupling  of  this  circuit 
to  the  secondary  system  merely  reduced  the  amplitude  of  the  locally 
generated  oscillations,  and  since  some  optimum  value*  of  the  locally 
generated  current  in  the  heterodyne  system  gives  the  best  amplification, 
this  was  asserted  to  be  the  effect  of  the  sensitizing  circuit. 

According  to  Captain  Armstrong,  the  key-note  to  the  phenomenon 
is  that  coupling  the  circuit  L-6,  0-6,  to  the  secondary  circuit  gives  this 
system  two  natural  frequencies  of  oscillation  differing  by  500  to  1,500 
cycles  per  second,  one  of  which  coincides  with  the  incoming  signal  and 
the  other  with  the  locally  generated  radio  frequency  current. 

That  this  circuit  will  amplify  the  incoming  signals  is  now  clear. 
If  the  valve  is  generating  oscillations  and  the  frequency  of  the  antenna, 
grid,  and  plate  circuits  is  adjusted  for  the  production  of  beats,  these 
circuits  are  slightly  out  of  resonance  with  the  incoming  signal  and  there- 
fore offer  some  reactance;  but  when  the  grid  circuit  possesses  two  de- 
grees of  freedom,  one  of  these  can  be  the  frequency  of  the  incoming 
signal  and  the  other  the  frequency  of  the  locally  generated  current. 
Hence,  the  circuits  possess  zero  reactance  to  either  the  local  frequency 
or  the  signalling  frequency  which  permits  either  current  to  rise  in  ampli- 
tude. The  same  effect  can  be  secured  by  close  coupling  of  the  open  and 
closed  circuits.  But  with  the  sensitizing  circuit  a  smaller  degree  of 
coupling  can  be  employed  between  the  open  and  closed  circuits  with  the 
same  strength  of  signals,  permitting  greater  freedom  from  interference. 

46.  RECEPTION  OF  CONTINUOUS  WAVES  BY  THE 
VALVE  WITHOUT  BEATS.— We  are  indebted  to  Captain  Arm- 
strong for  the  disclosure  of  a  circuit  for  the  reception  of  continuous 
waves,  which  employs  the  generating  feature  of  the  vacuum  valve,  but 
does  not  make  use  of  the  beat  phenomenon.  The  circuits  appear  in 
Figure  70. 

The  incoming  oscillations,  which,  of  course,  are  of  continuous  ampli- 
tude, are  varied  at  an  audio  frequency  by  setting  the  first  valve  into 
self -oscillation  at  an  audio  frequency.  Meanwhile,  incoming  radio  fre- 
quency currents  are  repeated  into  the  plate  circuit  of  the  first  valve  and 


*See  paragraph  52. 


106 


Vacuum  Tubes  In  Wireless  Communication 


Vacuum  Tubes  In  Wireless  Communication  107 

are  amplified  by  the  second  valve  through  regenerative  coupling,  but 
their  amplitude  is  modulated  at  an  audio  frequency  by  the  first  valve. 
Hence,  groups  of  radio  frequency  oscillations  flow  in  the  grid  circuit  of 
the  second  valve  where  rectificaton  takes  place  as  usual. 

It  is  to  be  observed  that  the  grid  potential  of  valve  No.  1  is  regu- 
lated by  battery  J5-3  and  potentiometer  P-l.  The  plate  and  grid  circuits 
are  coupled  through  an  audio  frequency  transformer  M-l,  the  primary 
and  secondary  coils  of  which  are  tuned  by  condensers  C-2  and  0-1 
respectively.  The  radio  frequency  component  of  the  plate  current  of 
the  first  valve  is  impressed  upon  the  grid  of  the  second  valve  through 
the  plate  inductance  LA. 

Tracing  the  connections  further,  the  second  valve  is  equipped  with  a 
radio  frequency  regenerative  transformer  M-3  (which  amplifies  the  in- 
coming signal),  and  the  complete  grid  circuit  includes  the  inductances 
L-5,  L-6,  and  L-l  and  variable  condenser  C-3.  A  path  for  the  radio 
frequency  current  flowing  in  the  output  circuit  around  the  telephone 
of  the  second  valve  is  provided  by  condenser  C-5. 

The  processes  involved  in  the  detection  of  undamped  oscillations 
by  the  circuit  of  Figure  70  follow:  the  first  valve  is  set  into  oscillation 
at  an  audio  frequency  through  the  coupling  M-l,  so  that  if  a  telephone 
were  inserted  in  the  plate  circuit  of  B-2,  a  continuous  tone  would  be 
heard  in  the  telephone  receiver.  Simultaneously  the  incoming  oscilla- 
tions impressed  upon  the  grid  circuit  of  the  first  valve  are  repeated  into 
its  plate  or  output  circuit  and  thence  impressed  upon  the  grid  or  input 
circuit  of  the  second  valve  through  coils  LA  and  L-5,  wherein  amplifi- 
cation and  detection  takes  place  in  the  usual  manner ;  that  is,  the  ampli- 
tudes of  the  incoming  radio  frequency  oscillations  are  varied  at  an  audio 
frequency,  and  therefore  the  potential  of  the  grid  varies  in  accordance. 

The  foregoing  actions  are  shown  graphically  in  Figure  71,  where  the 
oscillations  of  the  incoming  signals  are  indicated  on  the  line  0-1,  the 
modulated  radio  frequency  currents  by  the  line  0-2,  the  grid  oscillations 
of  the  second  valve  by  the  line  0-3,  and  the  current  in  the  output  or  tele- 
phone circuit  of  the  second  valve  by  the  line  0-4. 

In  practical  operation  it  is  essential  that  the  coupling  of  the  radio 
frequency  transformer  M-2  be  carefully  adjusted,  for  otherwise  the 
audio  frequency  component  of  the  plate  circuit  of  the  first  valve  will 
be  heard  as  a  musical  tone  in  the  telephone  of  the  second  valve. 

The  primary  and  secondary  coils  of  transformer  M-2  are  connected 
to  earth.  This  prevents  electrostatic  coupling  between  the  windings  LA 
and  L-5  through  the  telephone  of  the  second  valve  to  earth. 

A  simple  buzzer  placed  in  inductive  relation  to  circuits  of  the  first 


108 


Vacuum  Tubes  In  Wireless  Communication 


valve  obviously  would  give  approximately  the  same  results  as  this  cir- 
cuit.    The  audio  frequency  coupling  then  may  be  eliminated. 

47.  CASCADE  AMPLIFICATION.— The  output  circuits  of  any 
of  the  foregoing  types  of  beat  receivers  can  be  connected  to  a  battery 
of  vacuum  valves  in  cascade  for  further  amplification  of  the  incoming 
signal.  Either  the  radio  or  audio  frequency  component  may  ~be  ampli- 


0-1 


INCOMING  OSCILLATIONS 
FIRST  VALVE 


0-2 


MODULATED    PLATE 
CURRENT  FIRST 
VALVE 


0-3 


GRID  OSCILLATIONS 
SECOND   VALVE 


0-4 


TELEPHONE    CURRENT 
SECOND   VALVE 


Figure  71 

Indicating  the  phenomena  involved  in  the  operation  of  the  apparatus 
in  Figure  70. 


fied.  Cascade  operation  always  becomes  necessary  when  the  incoming 
signals  are  too  weak  to  effect  the  head  telephone.  The  student  who  has 
carefully  studied  previous  chapters  describing  the  various  cascade  cir- 
cuits will  be  able  to  devise  various  amplification  circuits. 

Better  results  are  obtained  under  the  usual  operating  conditions, 
by  connecting  the  valves  for  radio  frequency  amplification.  In  addition 
to  the  increased  strength  of  signals  resulting  therefrom,  increased  selec- 


Vacuum  Tubes  In  Wireless  Communication  109 

tivity  is  obtained,  that  is  to  say,  the  additional  energy  released  in  suc- 
cessive circuits  in  the  form  of  radio  frequency  oscillations  causes  these 
circuits  to  function  with  reduced  damping.  Captain  Armstrong  declares 
that  two  vacuum  valves  connected  in  cascade  for  radio  frequency  ampli- 
fication will  increase  the  incoming  signals  1,000  times. 

48  CASCADE  REGENERATIVE  SYSTEMS  FOR  BEAT  RE- 
CEPTION.— If  the  incoming  signals  are  too  weak  to  give  response  in 
the  plate  circuit  of  a  single  wave,  progressive  amplification  can  be  secured 
by  combining  the  regenerative  and  cascade  systems.  Such  systems  may 
be  employed  for  reception  of  continuous  or  discontinuous  waves. 

In  systems  of  this  kind  either  one  bulb  is  employed  to  amplify  and 
repeat  the  incoming  signal  into  a  second  bulb,  the  latter  being  set  into 
oscillation  for  the  production  of  beats,  or  the  circuits  of  both  bulbs  may 
be  set  into  oscillation.  Beats  then  will  be  generated  throughout  the 
entire  system;  but  in  general,  it  is  somewhat  difficult  to  keep  the  two 
circuits  oscillating  in  synchronism. 

If  the  two  bulbs  oscillate  out  of  synchronism  or  assynchronously, 
a  beat  note  is  obtained  causing  a  continual  hum  in  the  receivers,*  but 
by  carefully  adjusting  the  radio  frequency  tuning  of  one  of  the  valves 
the  beats  can  be  brought  into  synchronism  at  a  frequency  near  to  200 
cycles  per  second  as  is  evidenced  by  silence  in  the  head  telephones. 
Various  combination  circuits  are  possible.  One  circuit  is  shown  in 
Figure  72.  In  this  system,  amplification  in  the  first  valve  is  obtained  as 
usual : 

(1)  By  tuning  the  grid  and  plate  circuits  of  the  first  valve; 

(2)  By   carefully  adjusting  the   coupling  of  the   regenerative  trans- 

former M-l  for  maximum  amplification; 

(3)  By  adjusting  the  grid  potential  through  battery  B-3  and  the  grid 

potentiometer  P-l. 

The  principal  adjustments  for  radio  frequency  tuning  in  the  first 
valve  are  made  at: 

(1)  Antenna  inductance  L-l; 

(2)  Secondary  loading  inductance  L-3; 

(3)  Secondary  condenser  C-l; 

(4)  Plate  circuit  condenser  C-2. 

Similarly,  for  the  radio  frequency  circuits  of  the  second  valve  at: 

(1)  Secondary  loading  inductance  L-5; 

(2)  Secondary  condenser  C-5; 

(3)  Plate  circuit  condenser  C-6; 

(4)  Plate  inductance  L-4. 


*A  weak  beat  note  is  not  harmful  to  reception. 


no 


Vacuum  Tubes  In  Wireless  Communication 


Vacuum  Tubes  In  Wireless  Communication  111 

The  coupling  of  the  regenerative  transformer  lf-1  should  be  just 
close  enough  to  amplify  the  incoming  radio  frequency  signals.  The 
amplified  radio  frequency  component  of  the  plate  current  is  then  im- 
pressed upon  the  grid  circuit  of  the  second  valve  which  already  has  been 
set  into  oscillation  at  a  frequency  slightly  different  than  that  of  the 
incoming  signal. 

Beats  are  then  generated  in  the  second  system  which  are  rectified 
as  usual,  alternating  current  of  an  audio  frequency  flowing  through  the 
head  telephone  P-2.  Careful  adjustment  of  couplings  M-l,  M-2  and  M-3 
is  essential  for  stable  operation. 

It  may  be  mentioned  here,  that  the  primary  secondary  coils  of  the 
regenerative  transformers  generally  possess  fixed  values  of  inductance. 
Any  radio  frequency  tuning  which  may  be  necessary  in  either  circuit  is 
secured  through  additional  variable  inductances  or  condensers. 

It  tends  toward  simplicity  of  operation  in  cascade  systems  to  elimi- 
nate the  two  regenerative  couplings  of  Figure  72,  employing  instead 
the  simple  cascade  radio  frequency  amplifiers  shown  in  Figure  73. 

The  last  valve  of  the  group  may,  or  may  not  be  fitted  with  a  regen- 
erative coupling  M-3*  for  as  has  already  been  set  forth,  the  valve  cir- 
cuits may  oscillate  at  a  radio  frequency  without  regenerative  coupling. 

In  Figure  73,  L-2,  L-ll,  0-1  constitute  the  radio  frequency  tuning 
elements  of  the  grid  circuit  for  the  first  valve;  L-3,  0-2  for  the  plate 
circuit ;  L-4,  L-12,  0-3  for  the  grid  circuit  of  the  second  valve ;  L-5,  0-4 
for  the  plate  circuit  of  the  second  valve ;  L-6,  L-7,  L-8,  0-5  for  the  grid 
circuit  of  the  third  valve ;  and  0-6,  0-7,  L-9  and  Z/-10  for  the  plate  cir- 
cuit of  the  third  valve.  Battery  B-3  and  potentiometer  P-l  are  employed 
to  adjust  the  grid  potential  of  the  first  valve ;  similarly  B-5  and  P-2  for 
the  second  valve.  The  audio  frequency  reduction  and  increase  of  the 
plate  current  of  the  third  valve  is  secured  by  the  grid  condenser  C 
which  traps  the  rectified  beat  currents,  producing  a  relaying  action  on 
the  plate  current. 

It  will  be  seen  by  careful  consideration  of  the  circuits  in  Figure  73 
that  the  amplified  radio  frequency  component  of  the  plate  current  of  the 
first  valve  is  impressed  upon  the  grid  circuit  of  the  second  valve  through 
the  radio  transformer  M-l ;  similarly,  the  amplified  radio  frequency  com- 
ponent of  the  plate  current  of  the  second  valve  upon  the  grid  of  the 
third  valve  by  M-2. 

The  circuits  of  the  third  valve  are  set  into  self-oscillation  at  a 
slightly  different  frequency  than  that  of  the  incoming  signal  so  that  the 
interaction  of  the  two  currents  produces  I)  eats  which  are  rectified  and 


*See  paragraph  44. 


112 


Vacuum  Tubes  In  Wireless  Communication 


U'OOOWOO' 1 

^s t 


Vacuum  Tubes  In  Wireless  Communication 


113 


trapped  in  the  grid  condenser  C.  The  tuning  condensers  across  the 
primary  coils  of  if-1  and  M-2  can  be  eliminated,  sufficient  response  gen- 
erally being  secured  with  untuned  circuits. 

As  usual  a  special  resistance  JK-1  provides  the  requisite  leakage  for 
the  grid  condenser. 

It  here  should  be  borne  in  mind  that  coupling  two  valves  in  the 
manner  shown  results  in  the  withdrawal  of  energy  from  the  plate  cir- 
cuits of  the  first  valve,  and  some  readjustments  of  the  circuit  are  then 
required  for  maximum  response.  Such  adjustments  are  most  readily 
determined  by  experiment. 

49.  BEAT  RECEPTION  FROM  DAMPED  WAVE  TRANS- 
MITTERS.— Partial  beats  are  formed  when  locally  generated  oscilla- 
tions interact  with  discontinuous  or  damped  oscillations  such  as  those 
generated  by  spark  transmitters.  Due  to  the  discontinuity  of  the  radi- 
ated waves  complete  beat  formations  such  as  are  obtained  from  continu- 
ous waves  cannot  be  had.  Nevertheless,  amplification  results,  but  a  dis- 
torted note  is  obtained  in  the  receiving  telephone,  that  is,  the  normal 
note  of  the  spark  transmitter  is  destroyed  and  a  beat  note  of  a  rough 
quality  is  obtained. 


Figure  74 
Showing  the  use  of  a  simple  "tikker"  in  the  vacuum  tube  circuits. 

In  the  reception  of  damped  oscillations  (or  discontinuous  waves) 
through  a  receiver  responsive  to  either  type  of  oscillations  the  distor- 
tions due  to  beat  formations  can  be  prevented  by  careful  adjustment  of 
the  coupling  of  the  regenerative  transformer.  The  coupling  should  be 
close  enough  to  reinforce  the  plate  oscillations  through  the  grid  circuit 
but  not  to  set  the  valve  into  self -oscillation. 

50.  SPECIAL  CIRCUITS   FOR  THE  VACUUM   VALVE.— 

It  is  now  obvious  that  the  vacuum  tube  can  be  employed  to  amplify  the 
local  currents  of  any  type  of  radio  detector.  An  exemplary  circuit  is 
shown  in  Figure  74,  in  which  the  incoming  oscillations  are  interrupted 


114 


Vacuum  Tubes  In  Wireless  Communication 


at  an  audio  frequency  by  the  motor-driven  circuit  interrupter  or  tikker 
T.  In  this  way  the  amplitude  of  the  otherwise  continuous  oscillations 
impressed  upon  the  grid  circuit  is  varied  periodically  and  the  grid  con- 
denser may  therefore  be  energized  at  an  audio  frequency.  In  other 
words  a  system  for  the  detection  of  undamped  oscillations  is  here  pro- 
vided that  does  not  employ  regenerative  coupling  or  the  heterodyne 
principle. 

If  further  amplification  is  required,  a  number  of  valves  may  be 
connected  in  cascade  for  radio  frequency  or  audio  frequency  amplifi- 
cation. 

The  disadvantage  of  the  connection  shown  in  Figure  74,  is  that  the 
tikker  interrupts  the  conduction  current  from  the  grid  to  filament,  caus- 
ing a  humming  sound  in  the  telephone  independently  of  the  incoming 
signal.  But  this  may  be  prevented  by  placing  the  tikker  in  the  antenna 
circuit  or  in  a  special  circuit  to  which  the  valve  is  coupled  inductively. 


Figure  75 

Illustrating  the  use  of  the  revolving  variable  condenser  for  making 
audible  undamped  oscillations. 


(a)  The  Use  of  a  Rotary  Condenser  for  the  Reception  of  Con- 
tinuous Oscillations. — The  author  has  recently  taken  out  a  patent  on 
a  receiver  for  continuous  wave  reception,  the  fundamental  character- 
istics of  which  are  shown  in  Figure  75. 

A  revolving  secondary  condenser  (7-2  constructed  much  like  a  rotary 
spark  gap  is  connected  in  shunt  to  the  secondary  coil  of  the  receiving 
transformer  L-2.  This  condenser  contains  twelve  sets  of  stationary 
plates  mounted  in  a  circle  and  one  movable  set  driven  by  a  high-speed 
A.C.  or  D.C.  motor.  Now,  if  the  primary  and  secondary  circuits  of 
Figure  75  are  tuned  to  a  continuous  wave  transmitter  without  the  re- 
volving condenser  (7-2,  direct  current  will  pass  through  the  telephone  P; 


Vacuum  Tubes  In  Wireless  Communication 


115 


but  for  reasons  explained  in  paragraph  1,  the  telephone  diaphragm  will 
emit  no  sound.  If  the  condenser  rotates  at  such  speeds  to  throw  the 
secondary  system  into  and  out  of  resonance  200  to  1000  times  per  second, 
the  amplitude  of  the  telephone  current  will  be  varied  at  the  same  rate. 
Hence,  groups  of  direct  current  pulses  occurring  at  an  audio  frequency 
will  flow  through  the  head  telephone  P. 

The  advantage  of  this  method  over  the  usual  tikker  is  obvious;  all 
electrical  contacts  are  done  away  with  and  beyond  this,  the  condenser 
may  be  inserted  in  the  plate  or  grid  circuit  of  the  vacuum  valve  with- 
out actual  interruption  of  the  local  currents  which  would  produce  a 
humming  sound  in  the  telephone. 


HWh 
^wyvwl p-' 


Figure  76 

Special  circuit  for  the  revolving  condenser  in  connection  with  the 
reception  of  continuous  oscillations. 


The  author  has  had  some  success  with  the  connections  disclosed  in 
Figure  76,  where  the  amplitudes  of  the  incoming  oscillations  are  varied 
as  in  the  previous  system  by  the  rotating  condenser  (7-2  and  the  resulting 
groups  of  radio  frequency  currents  rectified  by  the  two  electrode  valve, 
F,  P.  They  are  thereafter  amplified  by  a  three  electrode  valve  F',  G',  P', 
the  grid  circuit  of  which  is  tuned  to  an  audio  frequency  by  the  shunt 
condenser  (7-3. 

The  two  electrode  valve  is  adjusted  to  the  most  favorable  working 
point  on  the  characteristic  curve  by  the  battery  B-2  and  the  potentio- 
meter P-l,  the  potential  of  B-2  varying  from  20  to  100  volts.  Groups 
of  rectified  currents  flow  through  winding  P  of  an  iron  core  transformer 
which  acts  inductively  upon  the  secondary  winding  8.  Through  potentio- 
meter P-2  and  battery  5-3,  the  potential  of  the  grid  to  filament  of  the 
three  electrode  valve  may  be  adjusted  for  the  best  amplification.  Further 


116  Vacuum  Tubes  In  Wireless  Communication 

amplification  may  be  secured  by  connecting  in  cascade  several  valves 
with  iron  core  transformers  inserted  between  the  output  and  input  cir- 
cuits of  successive  tubes.  The  revolving  condenser  C-2  obviously  can 
be  placed  in  any  part  of  the  radio  frequency  circuits  of  Figure  76. 

A  point  to  be  considered  in  the  operation  of  this  system  is  that  the 
condenser  must  be  constructed  so  that  its  capacity  as  the  plates  revolve 
covers  a  small  range,  for  otherwise,  signals  from  undesired  stations  will 
be  tuned  to  resonance  if  the  coupling  L-l,  L-2  is  close.  In  addition  care 
must  be  taken  to  select  the  proper  values  of  inductance  at  L-2,  so  that 
the  secondary  system  is  thrown  into  resonance  at  the  point  where  the 
revolving  condenser  reaches  its  maximum  capacity;  for,  otherwise,  the 
circuit  will  be  thrown  in  and  out  of  resonance  twice  for  each  set  of  sta- 
tionary plates,  and  a  rather  mixed  note  in  the  telephone  will  result. 

51.  "OPEN  CIRCUIT"  OSCILLATORS  FOR  THE  VACUUM 
VALVE. —  The  operating  characteristic  for  the  three-electrode  valve, 
shown  in  Figure  22,  indicates  that  the  strength  of  the  plate  current  at 
certain  points  along  the  characteristic  curve  varies  directly  with  the 
grid  potential.  Hence,  if  the  valve  be  employed  as  an  oscillation  de- 
tector, the  best  response  will  be  obtained  from  a  circuit  which  provides 
the  maximum  E.M.F.  for  a  given  group  of  oscillations.  This  condition 
is  satisfied  by  making  the  inductance  of  the  grid  circuit  very  large  and 
the  shunt  condenser  very  small  for  a  given  frequency  or  wave  length. 
In  general,  the  capacity  of  the  shunt  condenser  should  not  exceed  say 
.0005  microfarad. 

Over  the  range  of  the  shorter  wave  lengths,  the  secondary  con- 
denser may  be  dispensed  with,  the  distributed  capacity  of  the  secondary 
inductance  and  the  internal  capacity  of  the  vacuum  valve  (filament  to 
grid)  completing  the  oscillation  circuit. 

At  the  longer  wave  lengths  the  capacity  of  the  grid  circuit  induct- 
ance becomes  so  high  in  comparison  with  the  valve  capacity  that  better 
signals  are  secured  by  employing  a  secondary  condenser  to  fix  definitely 
the  maximum  and  minimum  potential  across  the  filament  and  the  grid. 

Because  of  the  high  potential  which  exists  on  the  free  end  of  an 
open  circuit  or  linear  oscillator,  a  coil  of  high  self-inductance  may  be 
employed  in  the  grid  circuit  of  the  vacuum  tube  to  amplify  the  signal. 
One  circuit  of  this  kind  devised  by  the  author  several  years  ago,  is  shown 
in  Figure  77,  where  the  usual  antenna  inductance  is  indicated  at  .L-l,  an 
open  circuit  or  grid  circuit  coil  at  L-2  being  placed  in  inductive  relation 
thereto  and  connected  to  earth  at  the  lower  end.  The  free  end  of  L-2 
connects  through  the  grid  condenser  C  to  the  grid  G  of  the  valve.  In 
event  that  the  length  of  the  coil  L-2  is  excessive  for  maximum  amplifi- 


Vacuum  Tubes  In  Wireless  Communication 


117 


cation,  it  may  be  reduced  and  shunted  by  a  variable  condenser.  A  unipo- 
lar connection  of  the  valve,  however,  is  still  used  as  shown  in  Figure  77. 


E-i 


Figure  77 

Open  circuit  oscillator  for  the  reception  of  damped  or  undamped 
oscillations. 


As  already  remarked,  amplification  is  obtained  in  this  circuit 
by  reason  of  the  increased  potential  impressed  upon  the  grid,  but  with 
very  long  coils,  the  resistance  losses  and  absorption  through  the  self- 
capacity  of  the  coil  may  be  excessive  and  the  selectivity  thus  afforded  may 
not  equal  that  of  a  smaller  coil  with  a  shunt  condenser.  The  coil  L-2 
obviously  must  be  of  considerable  length  for  the  longer  wave  lengths. 

The  natural  frequency  or  wave  length  of  open  circuit  coils  can  be 
measured  by  placing  a  wave  meter  set  into  excitation  by  a  buzzer  in 
inductive  relation  to  the  coil,  the  free  end  of  which  terminates  in  a  crystal 
rectifier  shunted  by  a  head  telephone. 

The  best  results  are  secured  with  this  system  when  L-2  is  of  differ- 
ent dimensions  for  each  wave  length.  In  other  words,  elimination  of  the 
unused  turns  is  desirable.  An  end  turn  switch  might  be  employed  to 
break  the  winding  into  groups  for  different  frequencies.  Such  a  switch, 
however,  does  not  eliminate  wholly  the  end  turn  losses,  but  it  aids  in 
this  direction.  For  complete  removal  of  these  effects  the  unused  turns 
must  be  removed  from  the  magnetic  field  of  the  used  turns. 

Coil  L-2  need  not  necessarily  be  connected  to  earth  at  the  lower 
end.  It  may  be  left  ungrounded,  but  it  must  then  possess  a  considerably 
greater  number  of  turns  for  a  given  wave  length,  than  when  conncted  to 
earth.  Its  natural  frequency  of  oscillation  must  equal  that  of  the  in- 
coming signal. 

Induction  from  low  frequency  power  circuits  is  prevented  by 
connecting  one  leg  of  the  filament  of  the  valve  to  earth.    The  earth  con- 


118 


Vacuum  Tubes  In  Wireless  Communication 


nection  is  not  essential  to  the  radio  frequency  tuning  of  the  secondary 
or  grid  circuit,  but  it  assists  in  eliminating  local  inductive  disturbances. 

(a)  Modified  Open  Circuit  Oscillators.— Numerous  modifications  of 
the  circuit  shown  in  Figure  77  are  possible.  For  example,  in  Figure  78 
the  valve  is  connected  to  the  free  end  of  an  aerial  Af  B,  which  is  con- 


Figure  78 

Novel   connection  of  the  three-electrode  valve.     The  grid  is   con- 
nected to  the  free  end  of  a  flat  top  aerial. 


L-z 


,8-2 


Figure  79 

Showing  the  use  of  open  circuit  oscillators  with  direct  coupling 
between  the  antenna  and  grid  circuits. 

nected  to  earth  through  the  tuning  inductance  L-l.  By  variation  of  L-l, 
the  maximum  potential  may  be  made  to  occur  at  the  free  end  of  the 
aerial,  which  is  attached  to  the  grid  G.  Amplification  is  thus  secured. 
A  circuit  much  similar  to  Figure  77,  preferred  by  the  author  is 
shown  in  Figure  79,  where  the  primary  and  secondary  circuits  of  the 


Vacuum  Tubes  In  Wireless  Communication 


119 


receiving  system  are  directly  coupled  and  the  grid  G  connected  to  the 
free  end  of  an  open  circuit  oscillator,  L-3.  Circuit  L-2,  L-3,  C,  G,  is  tuned 
to  the  frequency  of  the  incoming  signal  and  a  very  high  potential  is 
impressed  upon  the  grid.  The  filament  F,  as  usual,  is  connected  to 
earth  at  E-l.  Regenerative  couplings  may  be  employed  in  the  circuits  of 
Figures  77,  78,  or  79,  the  plate  circuit  being  coupled  to  the  grid  circuit 
at  L-3,  LA,  as  shown  in  Figure  80.  The  plate  circuit  is  tuned  to  reso- 
nance with  the  incoming  signal  through  inductance  LA  and  condenser 


L-4 


E-l 


Figure  80 

Modified  circuits  of  Weagant's  system  for  the  reception  of 
damped  or  undamped  oscillations.  The  plate  circuit  is  tuned  by  the 
inductance  L-4  and  the  condenser  C-2.  The  grid  is  attached  to  the  free 
end  of  an  open  circuit  oscillator. 

C-2  which  constitute  the  "X"  circuit  first  disclosed  by  R.  Weagant. 
This  circuit,  however,  will  respond  to  continuous  waves  without  the  re- 
generative coupling,  L-3,  LA.  (See  paragraph  44.) 

52.  AMPLIFICATION  BY  THE  HETERODYNE  SYSTEM.— 

The  numerical  degree  of  amplification  obtained  by  the  heterodyne  receiv- 
ing system  in  radio  has  been  a  point  of  considerable  debate  among  wire- 
less engineers.  Various  figures  have  been  claimed  varying  from  a  mini- 
mum of  four  to  several  thousand.  It  has  been  shown  mathematically  by 
Liebowitz  that  the  maximum  true  amplification  that  may  be  secured  by 
the  external  heterodyne  is  four,  and  that  any  increase  in  strength  of  sig- 
nals beyond  four-fold  amplification  is  due  to  some  increase  of  efficiency 
of  the  detector.  It  is  well  known,  however,  that  remarkable  amplifications 
are  secured  by  employing  the  vacuum  valve  detector  as  a  self -heterodyne. 


120  Vacuum  Tubes  In  Wireless  Communication 

In  fact,  the  total  amplification  as  actually  measured  has  totaled  several 
thousand. 

In  the  self-heterodyne  circuits  of  the  regenerative  type  where  the 
combined  functions  of  detection,  amplification,  and  generation  of  the  local 
radio  frequency  oscillations  occur  in  the  circuits  of  a  single  bulb,  there 
are  two  methods  of  amplification  which  take  place  simultaneously,  each 
one  operating  its  own  particular  way  practically  independent  of  the 
presence  of  the  other. 

With  the  point  in  view  of  separating  the  magnitude  of  the  various 
amplifying  effects  produced  by  the  self-heterodyne  as  compared  with  the 
simple  vacuum  valve  without  regenerative  circuits,  a  series  of  tests  was 
carried  out  by  Captain  Armstrong  at  a  wave  length  of  approximately 
8,000  meters.* 

The  first  measurements  were  for  the  purpose  of  comparing  the  sig- 
nal strength  obtained  with  a  simple  three-electrode  valve  coupled  to  an 
antenna  circuit  which  was  interrupted  approximately  600  times  per 
second  by  a  tikker  or  chopper,  and  the  signal  strength  secured  by  what 
Armstrong  terms  the  "equal  heterodyne."  This  is  the  term  he  applies 
to  the  condition  where  the  locally  generated  current  and  the  signalling 
current  are  equal  in  strength.  It  was  found  in  general  that  the  equal 
heterodyne  system  gave  a  signal  from  four  to  ten  times  as  loud  as  that 
given  by  the  tikker,  the  greatest  amplification  being  secured  on  the 
weaker  signals. 

In  the  second  series  of  tests,  the  locally  generated  radio  frequency 
current  was  increased  to  the  point  of  maximum  amplification  and  as  a 
matter  of  notation,  this  system  was  termed  the  "optimum  heterodyne." 
On  the  strongest  signals  the  response  for  the  best  adjustment  of  the 
locally  generated  current  was  approximately  one  and  one-half  times 
as  great  as  that  of  the  equal  heterodyne,  but  on  weaker  signals  amplifica- 
tion of  fifty-five  times  was  obtained. 

In  the  third  test,  measurements  were  made  for  the  purpose  of  de- 
termining the  relation  between  the  maximum  signal  strength  obtainable 
with  a  vacuum  valve  and  the  external  heterodyne  and  the  signal  obtain- 
able when  the  same  valve  is  coupled  as  a  self -heterodyne.  An  average 
amplification  of  approximately  fifty  times  was  secured. 

In  summary,  then,  the  equal  heterodyne,  the  optimum  heterodyne 
and  the  self-heterodyne  gave  amplifications  respectively  of  five,  twenty, 
and  fifty,  making  a  total  amplification  of  5,000  or  possibly  more. 

It  has  been  shown  by  Armstrong,  that  the  true  keynote  to  this  in- 


*See  Proceedings  of  the  Institute  of  Radio  Engineers,  Vol.  5,  No.  2,  April, 
1917. 


Vacuum  Tubes  In  Wireless  Communication  121 

crease  of  amplification  above  the  mathematically  established  factor  of 
four,  lies  in  what  may  be  called  the  "heterodyne  characteristic,"  i.e., 
the  relation  between  the  telephone  signal  strength  and  the  ratio  of  the 
local  to  the  signalling  current.  A  number  of  experimental  curves  were 
published  and  in  all  cases  beyond  the  1  to  1  point,  the  increase  of  the 
local  to  the  signalling  current  produced  a  very  rapid  increase  in  the 
telephone  signal  strength  which  rose  to  a  maximum  value  rather  rapidly 
and  then  fell  off  to  zero. 

The  rapid  rise  of  the  curve  as  the  local  current  is  increased  beyond 
the  1  to  1  point  is  due  to  the  shape  of  the  rectifying  or  valve  char- 
acteristic; that  is,  the  relation  between  the  grid  voltage  (with  respect  to 
the  filament)  and  the  grid  to  filament  current.  The  grid  current  is  the 
actual  conduction  current  flowing  from  grid  to  filament  and  it  is  upon 
the  amplitude  of  this  current  that  the  value  of  the  cumulative  charge  in 
the  grid  condenser  depends.  Two  curves  were  published,  one  showing  the 
grid- volt  age,  grid-current  characteristic  for  the  equal  heterodyne,  and 
the  other  for  the  optimum  heterodyne.  By  simple  analysis,  it  was  shown 
that  the  variation  of  current  is  obviously  very  much  greater  with  the 
optimum  heterodyne. 

53.     KENOTRON  AND  PLIOTRON  VACUUM  TUBES.— Dr. 

Irving  Langumuir  and  Saul  Dushman,  together  with  Mr.  William  C. 
White,  have  described  in  various  publications  the  two-electrode  and  three- 
electrode  vacuum  valve  tubes,  which  they  have  been  instrumental  in 
developing.  These  are  simply  extensions  of  Fleming's  original  dis- 
closures. The  two-electrode  valve  called  the  "Kenotron"  is  shown  in 
Figure  81,  and  the  three-electrode  valve  termed  the  "Pliotron"  is  shown 
in  Figure  82.  These  tubes  are  exhausted  to  an  exceedingly  high  vacuum 
by  an  elaborate  process.*  The  "Kenotron"  is  employed  principally  as  a 
rectifier  of  high  voltage  alternating  currents,  but  obviously,  small  bulbs 
might  be  employed  as  oscillation  detectors  for  the  reception  of  wireless 
telegrams. 

With  a  plate  potential  as  low  as  200  volts  electron  currents  up  to 
one  ampere  have  been  secured  with  these  tubes,  but,  as  pointed  out  by 
Langumuir,  a  more  serviceable  tube  is  one  designed  for  electron  currents 
of  250  milliamperes.  If  a  greater  current  is  desired,  several  "Kenotrons" 
may  be  operated  in  parallel. 

The  "Kenotron"  tubes  have  been  employed  to  rectify  alternating 
currents  up  to  180,000  volts.  For  any  purpose  for  which  high  voltage 
direct  current  is  desirable,  they  have  proven  practical  and  serviceable. 


*See  Proceedings  of  the  Institute  of  Radio  Engineers,  Vol.  3,  No.  3,  pp.  287- 
289,   Sept.,  1915. 


122 


Vacuum  Tubes  In  Wireless  Communication 


The  particular  tube  shown  in  Figure  81  is  designed  for  potentials  up  to 
50,000  volts.    As  a  rectifier  it  has  shown  an  efficiency  of  97.8%. 

In  the  three-electrode  tube  or  "Pliotron"  in  Figure  82,  the  grid  is 
shown  at  G,  the  filament  at  F — which  is  supported  by  a  glass  frame — 


PLATE      - 


FILAMENT 


PLATE 


Figure  81 

Showing    the   general    construction    of   the 
Kenotron  rectifier. 


and  the  plate  or  anode  at  A .  The  grid  consists  of  a  number  of  turns  of 
tungsten  wire  .01  millimeter  in  diameter,  spaced  to  include  100  turns 
tp  the  centimeter.  The  filament  is  mounted  inside  the  grid.  The  plates 


Vacuum  Tubes  In  Wireless  Communication 


123 


or  anodes  placed  on  either  side  of  the  filament  consist  of  tungsten  wire 
wound  zig-zag  in  the  manner  shown. 

A  large  "Pliotron"  which  may  be  termed  a  "power"  bulb  is  shown 
in  Figure  83.  A  tube  of  these  dimensions  may  be  employed  to  generate 
several  watts  of  radio  frequency  alternating  current  from  a  source  of 
direct  current.  Hundreds  of  bulbs  may  be  connected  in  parallel  for  the 
production  of  radio  frequency  currents  at  very  large  powers. 

A  battery  of  tubes  may  be  employed  to  control  the  output  of  a  radio 
frequency  alternator,  either  for  radio  telegraphy  or  telephony.  For 
example,  as  pointed  out  by  Dr.  Langumuir,  if  the  plate  is  connected  to 


Figure  82 

A  small  sized  Pliotron  for  use  as  an  oscillation  detector.     G  is  a 
tungsten  grid ;  F,  a  tungsten  filament  and  A,  the  anode  of  tungsten  wire. 

a  point  of  high  potential  in  the  antenna  system  (with  the  filament  con- 
nected to  earth)  and  the  grid  is  held  at  a  highly  negative  potential,  no 
leakage  of  the  antenna  current  takes  place,  But,  if  by  an  external  elec- 
tromotive force,  the  grid  potential  is  decreased,  sufficient  energy  may  be 
withdrawn  from  the  antenna  circuit  to  damp  out  the  oscillations.  Tele- 
graph signalling  may  thus  be  accomplished. 

Radio  telephony  may  be  carried  on  by  connecting  the  grid  and  fila- 
ment to  the  secondary  of  an  induction  coil,  the  primary  of  which  includes 
a  microphone  transmitter  and  a  battery.  Fluctuations  of  the  primary 
current  in  the  microphone  circuit  will  impress  upon  the  grid  of  the 
valve  a  vocal  current  of  several  hundred  volts  which,  in  turn,  will  vary 
the  amplitude  of  the  radio  frequency  currents  flowing  in  the  antenna 
circuit. 


124 


Vacuum  Tubes  In  Wireless  Communication 


In  the  curves  for  the  tube  of  Figure  82,  published  by  Dr.  Langumuir, 
approximately  26  volts  negative  grid  potential  reduces  the  plate  cur- 
rent to  zero.  A  positive  potential  of  10  volts  affords  a  plate  current  of 
nearly  ten  milliamperes.  With  the  grid  at  zero  potential,  plate  current 


FILAMENT -Y  — 


PLATE 


GRID 


Figure  83 
A  "power  bulb"  Pliotron. 

of  nearly  six  milliamperes  is  secured.  On  the  other  hand,  for  the  large 
"Pliotron"  shown  in  Figure  83  with  the  grid  potential  at  — 375  volts,  the 
plate  current  is  zero  and  at  — 105  volts,  the  plate  current  is  230  milli- 
amperes. 

Mr.  William  C.  White  has  disclosed  two  circuits  for  practical  use 


Vacuum  Tubes  In  Wireless  Communication  125 

of  the  '  *  Pliotron, ' '  one  for  the  production  of  high  voltage  radio  frequen- 
cies, and  the  other  for  the  production  of  low  voltage  radio  frequencies  at 
large  current  values. 

The  "Pliotron"  when  used  as  an  oscillator  for  the  production  of 
alternating  current  from  a  direct  current  source  of  energy  has  the  char- 
acteristics of  the  three-electrode  valve  as  already  discussed,  i.e.,  an  ampli- 
fying relay.  That  is,  the  wave  shape  of  any  variable  electromotive  force 
applied  between  the  filament  and  the  grid  will  be  faithfully  reproduced 
in  the  plate  circuit.  Therefore,  the  input  of  a  small  amount  of  alternat- 
ing current  energy  will  set  up  a  relatively  high  amount  in  the  plate 
circuit  identical  in  frequency  and  wave  shape.*  By  utilizing  a  small 
proportion  of  the  alternating  current  energy  thus  produced  to  feed  back 
to  the  grid  circuit,  this  system  can  be  made  self -exciting.  The  l '  Pliotron ' ' 
thus  becomes  a  converter  of  direct  currents  into  radio  frequency  alternat- 
ing currents  and  can  be  used  for  various  purposes  in  the  technical 
laboratory.  In  connection  with  the  circuit  of  Figure  84  for  the  produc- 
tion of  radio  frequency  currents  at  low  voltages,  Mr.  White  points  out 
first  that  in  a  resonance  circuit,  the  current  will  rise  until  the  losses 
become  equal  to  the  input  energy;  but  the  lower  limit  of  power  factor 
obtainable  in  practical  circuits  is  about  one-half  of  one  per  cent.  This 
permits  the  maximum  resonance  current  to  attain  a  value  of  about  two 
hundred  times  the  value  of  the  true  energy  current  fed  into  the  resonant 
circuit.  In  consequence,  if  large  currents  are  desired  from  a  small 
quantity  of  energy,  the  total  volt-amperes  in  the  circuit  must  be  kept 
small.  This  condition  requires  large  capacity  and  small  inductance. 

Again,  if  the  amount  of  electrical  energy  which  can  be  furnished  by 
a  certain  source  is  limited  by  the  amount  of  primary  power  available 
or  by  the  losses  in  transmission,  it  is  important  that  the  resistance  of 
the  load  be  adjusted  to  the  voltage  so  that  the  energy  will  be  economically 
utilized.  This  implies  in  the  case  of  the  vacuum  valve  circuit  shown  in 
Figure  84,  that  the  resistance  of  the  heavy  current  circuit  must  be  given 
the  apparent  value,  the  most  suitable  for  insertion  in  the  plate  circuit 
of  the  "Pliotron." 

(a)  The  Connections  of  the  Pliotron  for  the  Generation  of  Radio 
Frequencies  at  High  Current  Values.  In  the  circuit  shown  in  Figure 
84,  the  "Pliotron"  is  employed  to  generate  radio  frequency  currents  of 
high  current  value.  The  output  circuit  £-2,  C-2,  of  the  "Pliotron"  is 
coupled  to  the  heavy  current  circuit,  L-3,  (7-3  A,  A-1,  which  in  this 
particular  case  is  used  to  calibrate  ammeter  A  by  A-l  at  radio  frequencies. 


*This  is  true  of  any  properly  constructed  three-electrode  bull), 


126 


Vacuum  Tubes  In  Wireless  Communication 


The  grid  circuit  includes  the  inductance  L-I.    Condenser  C  provides  the 
requisite  electrostatic  coupling  to  keep  the  system  in  oscillation. 

It  is  important  to  carry  out  this  calibration  that  the  inductance  of 
L-3,  in  accordance  with  the  limitations  discussed  in  the  previous  para- 
graph, be  made  with  lowest  possible  amount.  Usually,  it  consists  of 
one  or  two  turns  of  heavy  conductor,  and,  therefore  the  capacity  of  0-3 
must  be  of  the  order  of  0.1  microfarad.  By  proper  adjustment  of  in- 
ductances L-l  and  L-2  and  variable  condenser  C,  the  "Pliotron"  will 
oscillate  energetically  at  a  radio  frequency  determined  by  the  electrical 


B  

Figure  84 

A  Pliotron  connected  up  for  the  production  of  continuous  oscilla- 
tions of  high  current  value  and  high  frequency. 


Figure  85 

The  circuits  of  the  Pliotron  for  the  production  of  high  voltages  at 
radio  frequencies.  The  apparatus  as  connected  in  this  diagram  will 
produce  potentials  up  to  12,000  volts  at  the  spark  gap  G  at  frequencies 
up  to  100,000  cycles  per  second. 


dimensions  of  the  grid  and  plate  circuits.  Because  of  the  relative  values 
of  the  inductances  L-2  and  L-3,  the  apparent  resistance  in  the  plate 
circuit  occasioned  by  the  coupled  calibration  circuit  is  considerably  multi- 
plied, but  it  is  not  sufficient  to  absorb  all  available  energy,  and  to  increase 
this  apparent  resistance  further,  a  variable  condenser  0-2  is  shunted 
about  L-2. 


Vacuum  Tubes  In  Wireless  Communication 


127 


The  current  flowing  in  the  heavy  current  output  circuit  in  Figure  84 
is  dependent  upon  the  voltage  of  the  direct  current  source.  The  most 
practical  range  has  been  found  to  be  between  200  and  750  volts.  Con- 
nected as  shown  in  this  drawing  the  "Pliotron"  will  generate  current 
at  frequencies  from  100,000  to  1,000,000  cycles  per  second,  and  by  oper- 
ating several  tubes  in  parallel  much  larger  current  outputs  may  be 
obtained. 

(b)  The  Pliotron  as  a  Generator  of  High  Voltages  at  Radio  Fre- 
quencies. A  circuit  for  the  production  of  high-voltage  radio  frequency 
currents  is  shown  in  Figure  85.  Here  the  inductances  L-\  and  L-2  are  of 
approximately  8  millihenries  each.  Inductances  L-3  and  LA  are  of  ap- 
proximately 2.5  millihenries  each.  The  latter  are  preferably  of  the 
variometer  type,  but  a  simple  coil  fitted  with  plug  contacts  satisfies  the 
requirements. 

The  plate  voltage  is  furnished  by  a  direct  current  generator,  D. 
A  high  voltage  condenser  in  series  with  a  hot-wire  ammeter  A  is  shown 


GRID,. 


.FILAMENT 


PLATE 


PLATE 


Figure  86 

Showing  the  construction  of  commercial  type  of 
three-electrode  vacuum  valve. 


128  Vacuum  Tubes  In  Wireless  Communication 

at  (7-1.  The  capacity  of  0-1  should  lie  between  20  and  200  micro-micro- 
farads for  a  frequency  of  100,000  cycles.  Knowledge  of  the  currents  flow 
at  A  and  the  frequency  of  the  circuit  permits  the  voltage  across  the 
condenser  0-1  to  be  simply  calculated. 

If  the  inductances  and  capacities*  are  properly  proportioned  for  a 
frequency  of  100,000  cycles  and  the  voltage  of  the  dynamo  lies  between 
200  and  750  volts,  voltages  up  to  12,000  may  be  secured  at  the  spark 
gap  G.  The  object  of  having  L-4,  Figure  85,  variable  is  for  the  purpose 
of  applying  the  high  frequency  energy  from  the  "Pliotron"  to  the 
resonance  circuit  at  the  correct  voltage  so  that  the  energy  available  is 
used  most  advantageously  in  the  resistance  of  this  circuit. 

A  commonly  used  type  of  three-electrode  valve  as  an  oscillation  de- 
tector is  shown  in  Figure  86.  The  material  of  the  grid,  plate  and  fila- 
ment being  as  follows:  The  filament  is  made  of  lime  coated  platinum 
and  is  energized  by  a  4  to  6  volt  battery;  the  grid  is  composed  of  a 
number  of  turns  of  tungsten  wrire  spaced  equally  on  either  side  of  the 
filament.  The  plates  are  made  of  sheet  nickel  J^"  by  1"  placed  on  either 
side  of  the  grid.  The  voltage  of  the  plate  circuit  varies  from  90  to  150 
volts  or  more. 


*Mr.  White  mentions  that  two  metal  plates  10"  by  10"  placed  approximately 
%"  apart  will  afford  a  condenser  having  capacity  of  approximately  40  micro- 
microfarads. 


PART  VII 
WIRELESS  TELEPHONY 

54.  In  General. —  The  art  of  wireless  telephony  has  been  so  aptly 
and  completely  covered  in  Dr.  Goldsmith's  "Radio  Telephony"  that  the 
subject  will  be  touched  upon  lightly  here.  The  vacuum  tube,  however, 
occupies  such  an  important  place  in  wireless  telephone  systems,  that  this 
volume  could  not  be  considered  complete  without  some  mention  of  cer- 
tain circuits  which  have  been  disclosed  by  various  investigators. 

In  general,  wireless  telephone  conversations  are  tranmitted  by  radio 
frequency  wave  motion  termed  the  carrier  wave.  This  carrier  wave  is 
modulated  at  an  audio  frequency  by  a  microphone  transmitter  such  as 
employed  in  land  line  telephony.  Thus,  any  undamped  wave  transmitter 
and  any  type  of  oscillation  detector  giving  a  quantitative  response  may 
be  employed  provided  a  magnetic  telephone  is  the  current  translator. 

Consider,  for  example,  a  wireless  transmitter  which  generates  a 
steady  wave  stream  of  continuous  amplitude.  If  a  wireless  receiving  set 
containing  a  simple  rectifier  and  a  telephone  is  tuned  to  this  trans- 
mitter, pulses  of  direct  current  in  rapid  succession  traverse  the  telephone 
windings  but  produce  no  sound.  But  if  the  amplitude  of  the  radiated 
energy  is  modulated  at  an  audio  or  vocal  frequency  through  a  micro- 
phone, the  amplitude  of  the  rectified  telephone  current  will  be  varied, 
at  a  vocal  frequency  in  accordance.  Hence,  the  diaphragm  of  the  tele- 
phone will  vibrate  at  the  same  rate  as  the  diaphragm  at  the  transmitter. 

The  audio  frequency  variations  occasioned  by  the  microphone  gen- 
erally occur  at  rates  from  100  to  2,000  per  second,  the  mean  average  being 
approximately  1,000  per  second.  This  average  value  has  been  termed  the 
speech"  frequency. 

The  great  problem  heretofore  in  radio  telephony  has  been  the  diifi- 

129 


130  Vacuum  Tubes  In  Wireless  Communication 

culty  of  modulating  the  large  powers  employed  at  the  transmitter  by 
the  usual  telephone  microphone  which  at  its  best  can  handle  approxi- 
mately from  one-half  to  one  ampere  of  current.  Owing  to  the  inherent 
imperfections  of  the  microphone  and  its  limited  current  carrying  capac- 
ity, a  continuously  operative  high  power  radio  telephone  system  was  not 
produced  until  the  advent  of  the  vacuum  tube. 

In  the  earliest  radiophone  systems,  a  number  of  microphones  were 
connected  in  parallel,  and  then  in  series  with  some  part  of  the  antenna 
system  or  in  special  circuits  associated  inductively  or  conductively  with 
the  antenna  system;  but  owing  to  the  "packing"*  of  the  microphone 
and  the  difficulty  of  over-heating,  only  very  small  powers  could  be  em- 
ployed. 

Various  types  of  high  power  microphones  have  been  constructed, 
but  they  cannot  be  said  to  have  satisfied  the  demands  of  modern  engi- 
neering, i.e.}  they  were  not  continuously  operative. 

Experiments  have  been  made  wherein  the  microphone  was  connected 
in  the  field  circuit  of  a  D.  C.  generator  supplying  current  to  an  arc  gen- 
erator, or  in  a  similar  way  to  alter  the  field  excitation  of  a  radio  fre- 
quency alternator;  but  such  systems  were  only  partially  successful,  be- 
cause of  the  small  current  carrying  capacity  of  the  microphone  and  the 
consequent  limited  degree  of  modulation  of  the  antenna  current. 

In  general,  we  may  state  that  the  systems  of  radio  telephony  so  far 
proposed  contemplate  the  generation  of  a  carrier  wave  above  the  limits 
of  audibility,  and  the  modulation  of  either  the  amplitude  or  the  wave 
length  of  the  wave  at  speech  frequency.  In  some  systems,  one  or  the 
other  function  is  performed  individually,  but  in  others  both  occur  to  a 
limited  degree  simultaneously. 

The  vacuum  tubes  seem  to  offer  a  practical  solution  of  this  problem, 
for,  as  already  explained,  they  can  be  employed  to  generate  radio  fre- 
quency alternating  currents  of  any  desired  frequency.  The  grid  potential- 
plate  current  characteristic  of  a  properly  constructed  valve  indicates  also 
that  a  very  slight  change  in  the  grid  potential  will  cause  a  relatively 
large  variation  of  the  plate  current.  Hence,  if  a  vacuum  valve  be  con- 
nected up  for  the  production  of  continuous  oscillations  and  a  micro- 
phone and  battery  be  connected  inductively  or  conductively  to  the  grid 
circuit,  the  grid  potential  will  rise  and  fall  in  accordance  with  the  sound 
modulations  of  the  human  voice  and  the  amplitude  of  the  radio  fre- 
quency carrier  wave  will  be  modulated  at  vocal  frequencies. 

(a)  Fundamental  Circuits  of  the  Three-Electrode  Tube  as  a 
Radiophone  Transmitter.  A  diagram  typifying  this  system  of  connec- 


*Coherence  of  the  carbon  granules. 


Vacuum  Tubes  In  Wireless  Communication 


131 


tions  is  shown  in  Figure  87.  The  grid  and  plate  circuits  of  the  vacuum 
tube  are  magnetically  coupled  at  -L-l  and  .L-2,  both  circuits  being 
tuned  to  a  given  frequency  of  oscillation  by  condensers  0-1  and  0-2  and 
the  inductances  L-l  and  L-2.  A  similar  circuit  has  been  shown  in  Figure 
60.  The  antenna  circuit  A,  L,  may  be  coupled  to  either  the  plate 
coil  L-2  or  the  grid  coil  L-l.  If  tuned  to  resonance,  considerable  amounts 


Figure  87 

Vacuum  valve  connected  up  for  the  production  of  continuous  oscil- 
lations for  radio  telephony. 


Figure  87a 

Modified  connection  of  the  microphone  to  the  valve  generator  for 
radio  telephony. 

of  energy  will  be  withdrawn  from  the  valve  circuits  into  the  antenna 
system  from  which  part  of  the  energy  is  radiated  in  the  form  of  electro- 
magnetic waves. 

An  induction  coil  M-l  with  the  primary  and  secondary  windings  P 


132 


Vacuum  Tubes  In  Wireless  Communication 


and  8  respectively  may  be  connected  to  the  condenser  GY-1,  the  primary 
circuit  of  the  coil  including  the  microphone  T  and  the  battery  B-3.  By 
proper  design  of  the  induction  coil,  two  or  three  hundred  volts  may  be 
impressed  upon  the  grid  and  a  very  great  modulation  of  the  antenna  cur- 
rent thus  secured.  Valves  employed  for  the  generation  of  radio  fre- 
quencies at  high  powers  have  grid  potentials  of  150  volts  negative,  and 
the  plate  potentials  may  attain  2,000  volts  or  more.  It  should  be  under- 
stood that  Figure  87  is  simply  a  suggestive  circuit.  It  may  be  re-drawn 
as  in  Figure  87a,  coils  L-l  and  L-2  being  considered  as  one  long  coil 
tapped  at  the  center  to  the  filament.  A  single  condenser  C-l  can  be 


Figure  87b 
Another  method  of  connecting  the  microphone  to  the  valve  generator. 


used  to  tune  the  plate  and  grid  circuits  simultaneously.  The  microphone 
and  its  induction  coil  may  be  connected  as  in  Figure  87a,  or  in  another 
way  as  in  Figure  87b,  where  a  large  condenser  (7-4  is  connected  across  the 
secondary  terminals  of  the  induction  coil  and  in  series  with  the  inductance 
of  the  grid  circuit. 

There  are  many  combination  circuits  for  wireless  telephony  in  which 
the  three-electrode  vacuum  tube  may  be  employed  in  one  way  or  the  other. 
For  example : 

(1)  A  number  of  bulbs  connected  in  parallel  may  generate  the  requi- 

site radio  frequency  current  and  one  or  more  additional  bulbs 
including  a  microphone  transmitter  may  be  used  to  vary  the 
output  of  the  "power  bulbs"  by  variation  of  their  grid  poten- 
tial. 

(2)  The  radio  frequency  current  for  the  carrier  wave  may  be  gener- 

ated by  a  radio  frequency  alternator  and  modulated  by  con- 
necting a  three-electrode  valve  at  some  effective  point  in  the 
antenna  system. 


Vacuum  Tubes  In  Wireless  Communication 


133 


(3)  A  battery  of  "power'*  vacuum  tube  bulbs  may  be  employed  to 

amplify  the  output  of  a  small  radio  frequency  alternator  and 
the  grid  potential  varied  at  mean  speech  frequency  by  addi- 
tional bulbs  including  in  their  grid  circuit  an  induction  coil 
and  a  microphone. 

(4)  A  battery  of  tubes  controlled  by  a  microphone  may  vary  the  field 

excitation  of  a  radio  frequency  alternator. 

(b)  Rounds'  Wireless  Telephone  System.  A  few  examples  of  pro- 
posed systems  will  be  described.  The  first  system  to  employ  the  vacuum 
valve  as  a  source  of  oscillations  for  wireless  telephony  was  that  developed 
by  H.  J.  Rounds  of  Marconi's  Wireless  Telegraph  Company,  Ltd.,  which 
is  shown  in  Figure  88.  It  will  be  noted  in  this  diagram  that  the  grid  and 
plate  circuits  of  a  vacuum  tube  are  coupled  at  L-3  and  LA,  the  antenna 
being  coupled  to  the  grid  circuit  at  L-l.  The  plate  battery  B-2  varying 


Figure  88 

Rounds'  system  of  wireless  telephony.  Continuous  oscillations  are 
generated  in  the  circuits  of  the  tube  F',  G',  P',  transferred  to  the  antenna 
through  the  coupling  L-l,  L-3,  and  modulated  by  the  microphone  T. 

from  500  to  2,000  volts  is  shunted  by  the  condenser  0-2.  Four  resistances, 
R-l,  #-2,  R-3— of  500  ohms— and  K-4  of  10,000  ohms,  are  connected  in 
series  with  the  plate  battery. 

The  filament  battery  is  an  80  ampere  hour  6  volt  cell,  and  the 
grid  battery  B-3  has  a  voltage  of  approximately  500  volts.  A  microphone 
T ,  is  connected  in  series  with  the  antenna  circuit  as  well  as  a  small  glow 
lamp  N,  which  is  employed  to  indicate  conditions  of  resonance  between 
the  generating  circuit  and  the  antenna  circuit. 

The  entire  system  is  set  into  oscillation  by  opening  and  closing  the 
key  connected  in  the  grid  circuit.  Resonance  is  established  by  careful 
adjustment  of  condensers  0-1  and  0-2.  By  further  adjustment  of  the 
reaction  coupling  and  by  tuning  the  antenna  circuit,  the  small  glow 
lamp  N,  will  light  to  full  brilliancy.  This  indicates  that  the  antenna 


134  Vacuum  Tubes  In  Wireless  Communication 

circuit  is  in  a  state  of  oscillation,  that  is,  N  takes  the  place  of  the  antenna 
ammeter.  When  the  transmitter  T  is  spoken  into,  the  radio  frequency 
wave  is  modulated  at  speech  frequency  and  response  will  then  be  secured 
at  the  receiving  station.  Note  should  be  made  of  the  fact  that  the  micro- 
phone as  here  connected  in  the  circuit  is  not  in  the  most  effective  position, 
but  later  circuits  developed  by  Rounds  show  the  microphone  connected 
in  some  part  of  the  grid  circuit.  With  the  connections  of  Figure  88, 
radio  telephony  was  accomplished  over  distances  up  to  50  miles. 

55.  HUND'S  METHOD  OF  RADIO  FREQUENCY  MODU- 
LATION.—  A  method  for  controlling  the  carrier  wave  at  an  audio  or 
vocal  frequency  suggested  by  August  Hund,  is  disclosed  in  Figure  89. 
In  this  system,  the  antenna  oscillations  are  modulated  at  vocal  frequency 
by  a  three-electrode  vacuum  tube.  The  radio  frequency  currents  for  the 
carrier  wave  are  generated  by  the  vacuum  tube  V,  the  grid  and  plate  cir- 
cuits being  coupled  together  as  usual  for  the  generation  of  radio  fre- 
quency currents.  By  coupling  L-2  to  L-l,  currents  of  similar  frequency 
are  induced  in  the  antenna  circuit. 

The  antenna  system  further  includes  the  coil  L-3  which  may  have 
from  6  to  15  microhenries  inductance.  The  terminals  of  L-3  are  con- 
nected to  the  plates  P-l  and  P-2  of  the  three-electrode  bulb  V-1. 

The  filament  F-I  is  rendered  incandescent  by  the  battery  B-l  and 
the  potential  of  the  grid  to  filament  varied  at  speech  frequency  by  the 
microphone  T  through  the  iron-core  induction  coil  P-l,  $-1.  The  poten- 
tial of  the  grid  in  respect  to  the  filament  can  be  maintained  at  the  most 
satisfactory  value  by  battery  B-3  and  potentiometer  P.  The  grid  nor- 
mally is  held  at  a  fairly  high  negative  potential  so  that  no  currents  leak 
around  L-3  through  the  valve  "P-l;  but  when  the  microphone  T  is 
spoken  into,  the  valve  becomes  conductive  at  vocal  frequencies-;  one- 
half  cycle  of  the  carrier  wave  leaks  through  the  conduction  path  from 
P-l  to  F-l,  and  similarly  the  other  half  cycle  through  the  conduction  path 
from  P-2  to  F.  Energy  is  thus  withdrawn  from  successive  cycles  of  the 
carrier  wave  in  accordance  with  the  vocal  wave  impressed  upon  the 
grid  by  the  microphone  T  and  the  transformer  P-l,  $-1.  In  summary, 
the  grid  potential  is  modulated  by  the  microphone,  and  the  antenna  cur- 
rent leaks  from  plates  P-l  and  P-2  to  filament  F-\.  The  antenna  oscilla- 
tions are  damped  out  at  speech  frequency. 

Obviously,  two  valves  might  be  employed  to  secure  this  leakage. 

56.    ENGLUND'S   WIRELESS    TELEPHONE    SYSTEM.— It 

has  been  established  that  if  the  frequency  of  the  carrier  wave  in  wire- 
less telephony  is  of  constant  amplitude  and  F  cycles  per  second,  and  it 


Vacuum  Tubes  In  Wireless  Communication 


135 


is  modulated  at  a  vocal  frequency  /  such  as  generated  by  a  microphone, 
there  will  be  radiated  into  the  ether  a  complex  wave  which  is  made  up 
of  three  component  frequencies  f,  F  —  /,  and  F  -\-  f.  It  is  evident  that 
the  unmodulated  component  of  the  antenna  current  F  since  it  does  not 
contain  the  signal  frequency  /,  need  not  be  present  in  the  antenna,  but  it 
may  be  supplied  by  an  auxiliary  source  at  the  receiving  station.  Hence,  if 
by  any  means  the  carrier  frequency  F  can  be  eliminated,  except  as  at  such 


MODULATOR 


Figure  89 

Hund's   proposed   method   for  modulating   radio   frequency   currents 
by  a  microphone. 

times  that  the  wireless  telephone  transmitter  is  spoken  into,  a  considerable 
wastage  of  energy  is  prevented,  and,  furthermore,  the  carrier  frequency 
F  will  not  i  terfere  with  the  operation  of  other  stations  in  proximity. 

In  Engiund's  proposed  system,  shown  in  Figures  90*  and  91*,  this 
phenomenon  is  taken  into  consideration  and  means  have  been  devised 


*It  cannot  be  said  that  systems  outlined  in  these  diagrams  have  attained 
the  stage  of  practical  commercial  application.  They  are  merely  published  to 
indicate  the  general  trend  of  experimentation. 


^-S a 

a  9M  3  S 

a  -2  »  .  0  -s 


•rH    (jj  "^    3    r* 

llfll 

-M    53    P    "-1    P       • 

llfPl 


w 


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'oJ   >  &   Q   O 


Vacuum  Tubes  In  Wireless  Communication 


137 


whereby  the  antenna  circuit,  Figure  90,  A,  L-13,  E,  is  traversed  by  modu- 
lated currents  only  when  the  transmitter  "T"  is  spoken  into.  From  this 
diagram,  the  student  receives  some  hint  of  the  fundamental  circuits  of  a 
long  distance  wireless  telephone  system  in  which  vacuum  valves  are 
employed  not  only  to  generate  the  radio  frequency  current  but  to  modu- 
late it  at  speech  frequency  as  well. 

It  is  to  be  noted  first,  that  the  plate  and  grid  circuits  of  the 
vacuum  tube  V-1  called  the  master  oscillator,  are  coupled  through  the  coil 
L-l  and  condenser  (7-1  for  the  production  of  radio  frequency  currents, 
that  is,  the  grid  and  plate  circuits  are  coupled  in  such  a  way  as  to 


L-15 


Figure  91 

Receiving  apparatus  of  Englund's  system. 
is  supplied  by  a  local  generator  A-l. 


The  carrier  frequency 


set  the  circuits  of  T7-!  into  oscillation.  Coupled  to  L-l  is  the  secondary 
coil  L-2  which  in  turn  is  connected  to  the  grid  and  filament  of  another 
vacuum  tube  V-2  termed  the  "modulator." 

The  output  circuit  of  the  " modulator"  bulb  is  coupled  to  L-6 
through  L-5.  The  circuit  of  L-6  in  turn  contains  a  bridge  consisting  of 
resistances  R-l,  R-2,  inductance  L-7,  and  condenser  (7-7.  Tapped  across 
this  bridge  is  the  inductance  L-8  coupled  to  L-9,  the  terminals  of  which 
in  turn  are  attached  to  the  grid  and  filament  (the  input  circuit)  of  a 
battery  of  tubes  F-3. 

By  properly  balancing  the  bridge,  no  current  flows  through  L-8  at 
the  carrier  frequency  F,  but  currents  of  a  frequency  differing  from  that 
of  the  carrier  frequency  destroy  the  balance  according  to  the  frequencies 
of  the  vocal  currents  generated  by  the  human  voice.  The  modulated  cur- 
rents are  then  amplified  by  the  vacuum  tubes  V-3  connected  in  parallel. 

The  output  circuits  of  these  bulbs  are  in  turn  coupled  to  the  grids 
and  filaments  of  the  battery  of  power  bulbs  VA  of  which  there  may  be 


138  Vacuum  Tubes  In  Wireless  Communication 

any  number  connected  in  parallel.     The  plate  circuits  of  VA  may  be 
fed  by  a  direct  current  dynamo  or  a  large  storage  battery.     Their  out- 
put circuits  are  in  turn  connected  to  the  antenna  system  at  L-12  and  L-13. 
Beginning  at  the  extreme  left-hand  part  of  the  drawing,  Figure 

90,  a  circuit  will  be  seen  containing  the  microphone  "T,"  a  battery  B, 
and  an  induction  coil  L-3,  L-4,  which  is  coupled  to  the  input  circuit  of 
the  modulator  bulb  V-2.    The  radio  frequency  current  modulated  by  the 
microphone  unbalances  the  bridge  network  above  mentioned  and  the 
resulting  currents  are  amplified  through  the  batteries  of  bulbs  V-3  and 
V-4.    The  oscillator  F-l,  of  course,  can  be  replaced  by  a  small  radio  fre- 
quency alternator. 

Very  feeble  currents  flowing  through  microphone  T  can  control 
antenna  current  of  many  kilowatts,  thus  greatly  increasing  the  distances 
over  which  wireless  telephony  can  be  carried  on. 

Since  the  carrier  wave  of  frequency  F  has  been  eliminated  by  this 
process,  it  must  be  supplied  at  the  receiving  station.  The  diagram,  Figure 

91,  shows  a  radio  frequency  alternator  A-l  (for  which  may  be  substituted 
a  vacuum  valve  connected  up  for  the  production  of  radio  frequency  os- 
cillations) coupled  inductively  to  the  antenna  system  at  L-16  and  L-14. 
A  vacuum  valve  V-5  is  employed  for  purposes  of  detection.    It  is  induc- 
tively coupled  to  the  antenna  circuit  through  coil  L-15. 

The  inventor  claims  that  in  addition  to  the  elimination  of  the  waste 
of  energy  by  this  system  there  is  a  slight  improvement  in  the  quality 
of  received  signals  due  to  the  elimination  of  the  frequency  F. 

57.  CARSON'S  PROPOSED  WIRELESS  TELEPHONE  SYS- 
TEM.—  Another  system  has  been  disclosed  by  John  Carson  in  Figure 

92,  in  which  the  antenna  does  not  radiate  except  when  the  microphone  is 
spoken  into.    It  is  to  be  noted  in  this  diagram  that  the  field  current  of 
a  radio  frequency  alternator  A  is  modulated  at  vocal  frequencies  by  a 
microphone  T,  the  currents  of  which  are  amplified  by  a  vacuum  tube  V. 
The  field  windings  F  of  the  alternator  are  thus  excited  at  speech  fre- 
quency and  the  amplitude  of  the  radio  frequency  current  generated  by  A 
varied  accordingly. 

The  complete  microphone  circuit  includes  the  transmitter  T,  the 
battery  B,  and  the  induction  coil  P,  S.  Winding  8  is  connected  to  the 
grid  or  input  circuit  of  a  three-electrode  valve  F,  the  plate  or  output 
circuit  of  which  includes  the  primary  winding  of  an  induction  coil  P-l 
coupled  to  the  secondary  winding  $-1.  The  secondary  circuit  includes  the 
condenser  C  and  the  field  winding  of  the  alternator  F.  The  armature  of 
the  alternator  A  is  connected  to  the  grid  or  input  circuit  of  the  tube  F-l 
whose  plate  circuit  0-1,  L-l,  may  be  fed  with  direct  current  from  battery 


Vacuum  Tubes  In  Wireless  Communication 


139 


or  direct  current  dynamo.  This  tube  repeats  and  amplifies  the  radio 
frequency  currents  impressed  upon  its  input  circuit  and  the  oscillations 
in  the  output  circuit  are  transferred  to  the  antenna  through  the  coupling 
L-l  and  L-2.  The  impedance  of  the  circuit  S-l,  C,  F  to  the  vocal  cur- 
rents is  reduced  by  condenser  C. 

In  summary,  the  modulated  currents  of  the  microphone  circuit  T, 
P,  B,  are  amplified  by  bulb  V,  and  a  fluctuating  current  circulates 
through  field  winding  F  which  varies  the  output  of  the  radio  frequency 
alternator  A  at  a  vocal  frequency.  A  current,  the  reproduction  of  the 
signalling  current,  therefore  flows  in  the  antenna  which  radiates  only 


Figure  92 


Carson's  system  for  wireless  telephony.  The  output  of  a  radio  fre- 
quency alternator  A  is  varied  at  vocal  frequency  through  a  microphone 
T  and  an  amplifier  bulb  V.  This  in  turn  varies  the  current  input  in  the 
field  windings  F  of  the  alternator. 


when  the  microphone  is  in  operation.  The  amplitude  of  the  high  fre- 
quency wave  radiated  from  the  antenna  is  directly  proportional  to  the 
low  frequency  signalling  wave,  hence  the  telephone  diaphragm  at  the 
receiving  station  is  deflected  at  vocal  frequencies. 

In  ordinary  systems,  as  we  have  already  mentioned,  there  is  a  con- 
tinuous radiation  in  the  form  of  an  unmodulated  carrier  wave  even  when 
the  transmitter  is  not  spoken  into.  The  transmission  of  this  wave,  be- 
sides involving  a  waste  of  energy,  constitutes  a  serious  bar  against  the 
operation  of  duplex  systems. 

58.  ESPENSCHIED'S  DUPLEX  WIRELESS  TELEPHONE 
SYSTEM. —  Among  the  attempts  that  have  been  made  to  secure  simul- 
taneous transmission  and  reception  in  wireless  telephony,  the  system 
evolved  by  Lloyd  Espenschied  is  of  interest.  A  problem  of  considerable 
magnitude  is  encountered  in  duplex  transmitting  and  receiving  systems 


140 


Vacuum  Tubes  In  Wireless  Communication 


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Vacuum  Tubes  In  Wireless  Communication  141 

because  of  the  large  amounts  of  power  used  for  transmitting  compared 
to  that  flowing  in  the  receiving  systems,  the  ratio  being  approximately 
one  million  to  one.  This  inventor  believes  he  has  solved  the  problem 
through  the  use  of  specially  devised  balancing  out  circuits. 

The  embodiments  of  Espenschied's  system  are  shown  in  the  diagram 
of  Figure  93,  additional  circuits  being  shown  in  Figures,  94  95  and  96. 
In  the  systems  shown  in  these  diagrams  the  inventor  secures  duplex 
operation  by  employing  different  carrier  frequencies  for  transmission 
and  reception.  Through  the  selectivity  thus  afforded  and  by  the  aid  of 
additional  balancing  out  circuits,  either  the  same  aerial  or  two  different 
aerials  may  be  employed  for  simultaneous  transmission  and  reception. 

In  brief,  the  antenna  system  shown  in  Figure  93,  comprises  two 
parallel  branches  B  and  C  which  gives  the  complete  system  two  natural 
frequencies  of  oscillation.  Branch  B  is  coupled  to  a  continuous  wave 
generator  A-l,  and  branch  C  is  coupled  to  a  valve  amplifying  system  in- 
cluding the  tubes  V-l  and  "7-2.  The  speech  signals  are  translated  through 
the  medium  of  the  telephone  T  connected  in  the  output  circuit  of  the 
tube  F-2. 

Keeping  in  mind  the  enormous  volume  of  energy  flowing  in  the  trans- 
mitting branch  compared  to  that  in  the  receiving  branch,  it  is  clear  that 
some  means  of  balancing  out  the  effect  of  branch  B  upon  the  branch  C 
must  be  employed.  This  is  accomplished  by  the  balancing  out  circuit 
8,  0-1,  0-2,  P-l.  8  is  coupled  to  the  radio  frequency  generator  A-l  and 
to  the  input  side  of  the  three-electrode  valve  at  P-l,  8-1.  By  proper  ad- 
justment of  the  phase  relation  of  the  balancing  out  current  and  the  cur- 
rent of  similar  frequency  induced  in  the  receiving  system,  complete  an- 
nulment is  secured  in  the  branch  0.  It  must  be  remembered  that  the 
frequency  of  the  balancing  out  circuit  is  that  of  the  transmitter.  Hence, 
only  currents  of  this  frequency  are  suppressed  in  the  receiving  system, 
leaving  it  free  to  receive  waves  at  a  frequency  differing  from  that  of  the 
radio  frequency  alternator  A-\.  Careful  adjustments  of  the  couplings 
P,  8,  and  P-l,  $-1,  are  essential  for  successful  operation. 

The  correct  phase  relation  between  the  balancing  currents  is  obtained 
by  proper  adjustment  of  capacity  of  the  condensers  0-1  and  0-2. 

The  circuit  shown  in  Figure  94  is  in  all  respects  similar  to  Figure 

93  with  the  exception  that  the  balancing  out  circuit  includes  a  vacuum 
tube  "F-3  which  amplifies  the  effect  of  generator  A-l.    Better  balance  of 
the  opposing  E.M.F.'s  is  thus  secured. 

It  is  thus  seen  that  in  a  general  way  the  circuits  of  Figures  93  and 

94  simulate  the  circuits  of  wire  telephony,  the  apparatus  always  being 
in  a  position  to  transmit  and  receive. 

A  system  involving  the  use  of  separate  aerials  for  transmission  and 


142 


Vacuum  Tubes  In  Wireless  Communication 


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Vacuum  Tubes  In  Wireless  Communication  143 

reception  of  speech  signals  disclosed  by  Espenschied  is  shown  in  Figure 
95.  The  aerial  of  the  transmitter  is  indicated  at  W,  and  of  the  receiv- 
ing station  at  W-l.  The  source  of  radio  frequency  current  for  the  carrier 
wave  is  shown  at  A-l,  the  output  of  which  is  amplified  by  means  of  the 
three-electrode  vacuum  tubes  V-l  and  "F-2,  the  output  currents  of  the 
latter  tube  being  fed  to  the  aerial  W  at  the  coupling  P9  S. 

The  receiving  system  embraces  the  coupling  transformer  P-l,  $-1,  the 
incoming  signal  being  amplified  by  the  three-electrode  tube  F-3  and 
detected  by  the  tube  F-4.  The  output  circuit  of  Y-4  includes  the  receiv- 
ing telephone  T. 

Through  the  transformer  M  and  the  microphone  T-l,  currents  of 
vocal  frequency  are  impressed  upon  the  circuit  X  which  also  is  induc- 
tively coupled  at  M-l  to  the  alternator  A-l.  The  output  of  the  alternator 
is  modulated  at  vocal  frequency  by  T-l.  The  circuit  X  is  coupled  to 
antenna  W  at  M-3.  Radio  frequency  current  is  thus  withdrawn  from 
the  antenna  circuit  through  the  circuit  X  coupled  to  V-3  at  M-2,  to  bal- 
ance out  currents  of  similar  frequency  in  the  receiving  system.  This  cir- 
cuit thus  serves  to  impress  currents  of  speech  frequency  upon  the  alter- 
nator A-l  and  to  deliver  radio  frequency  currents  to  the  input  cir- 
cuit of  the  tube  F-3  to  balance  out  such  currents  as  may  be  induced  in 
the  aerial  W-l  by  W.  The  correct  phase  relation  of  the  opposing  radio 
frequency  currents  is  obtained  by  careful  adjustment  of  condensers  0-1 
and  0-2. 

In  respect  to  the  reception  of  signals,  it  is  seen  that  antenna  W-l  is 
strongly  responsive  while  antenna  W  is  weakly  responsive  to  the  distant 
transmitter  owing  to  the  difference  of  frequency. 

Summarizing  the  actions  of  the  apparatus  disclosed  in  Figure  95, 
currents  of  radio  frequency  generated  by  the  radio  frequency  alternator 
^.-1  are  amplified  by  a  battery  of  vacuum  valve  tubes  the  output  cir- 
cuits of  which  are  inductively  coupled  to  the  antenna.  Circuit  X  serves 
to  conduct  radio  frequency  current  from  the  transmitter  for  balancing 
out  the  effects  of  the  transmitter  upon  the  receiving  system.  It  acts 
also  as  a  carrier  of  the  vocal  currents  generated  by  the  microphone  T-l. 

The  system  shown  in  Figure  96  fundamentally  is  similar  to  that 
of  Figure  95.  The  output  of  the  radio  frequency  alternator  A-l  is 
amplified  by  the  bulb  V-l,  the  carrier  wave  being  modulated  at  a  radio 
frequency  by  the  microphone  T-l  through  the  coupling  M.  The  output 
circuit  of  V-l  is  coupled  to  the  input  circuit  of  the  valve  V-2,  the  out- 
put circuit  of  which  is  inductively  coupled  to  the  antenna  through  the 
transformer  P,  8.  A  balancing-out  circuit  shunted  across  circuit  X  in- 
cluding the  condenser  0-1  and  the  coupling  M-2  serves  to  impress  a  modu- 


144 


Vacuum  Tubes  In  Wireless  Communication 


Vacuum  Tubes  In  Wireless  Communication 


145 


146  Vacuum  Tubes  In  Wireless  Communication 

lated  radio  frequency  current  on  the  input  circuit  of  the  detection  tubes 
y-3,  T-4.  Thus  currents  of  the  transmitter  frequency  which  may  be 
induced  in  the  antenna  W-l  are  balanced  out  leaving  the  receiving  sys- 
tem free  to  respond  to  waves  of  a  frequency  differing  from  that  employed 
in  the  antenna  system  W.  Correct  phase  relation  of  the  opposing  cur- 
rents is  obtained  by  means  of  the  condenser  0-1 

59.  ENGLUND'S  DUPLEX  RADIO  TELEPHONE  AND 
RADIO  TELEGRAPH  SYSTEM.— We  have  remarked  in  paragraph 
56  how  a  vocal  wave  or  current  of  speech  frequency  impressed  upon  a 
radio  frequency  or  carrier  wave  sets  up  three  complex  waves  of  fre- 
quencies F  -{-  f,  F,  and  F  —  /,  in  which  F  is  the  frequency  of  the  carrier 
wave  and  /  the  vocal  wave  impressed  upon  the  carrier  wave  by  the 
human  voice  through  a  microphone.  Because  the  wave  of  frequency  F 
does  not  contain  the  signal  frequency,  /,  it  represents  a  waste  of  power  in 
the  antenna  system.  Means  were  shown  (in  paragraph  56)  whereby  the 
current  of  frequency  F  could  be  practically  eliminated  at  the  transmitter 
but  be  supplied  at  the  receiver  by  a  local  generator. 

Englund  has  recently  disclosed  a  novel  system  which  not  only  em- 
bodies the  foregoing  principle,  but  in  which  the  frequency  F  is  employed 
for  telegraphic  signalling.  That  is,  the  antenna  is  used  for  simultaneous 
radiation  of  telephonic  and  telegraphic  messages. 

An  important  feature  of  the  system  is  the  fact  that  telegraphic  and 
telephonic  signals  may  be  dispatched  simultaneously  at  the  same  wave 
length  from  one  aerial  and  may  be  received  upon  one  aerial  at  the  re- 
ceiving station.  The  transmitting  circuits  of  this  system  are  shown  in 
Figure  97  and  the  receiving  circuits  in  Figure  98.  Beginning  at  the 
left-hand  side  of  the  drawing  of  Figure  97,  a  radio  frequency  alternator 
A  is  coupled  to  the  input  circuit  B  of  a  modulator  bulb.  Coupled  to  the 
same  input  circuit  is  a  microphone  circuit  0  including  the  microphone  T, 
the  battery  5-3,  and  the  transformer  M-2. 

The  output  circuit  of  the  modulator  bulb  contains  two  branch  cir- 
cuits B-l  and  B-2.  The  branch  B-~L  comprising  the  inductance  and  the 
condenser  serves  as  a  short  circuit  to  current  of  the  frequency  of  the 
generator  A.  The  parallel  circuits  of  branch  B-2  are  tuned  to  offer  a 
practically  infinite  impedance  to  currents  of  the  generator  frequency 
and  a  low  impedance  to  currents  whose  frequencies  differ  therefrom  by 
a  vocal  frequency. 

Through  the  transformer  M}l  currents  of  vocal  frequency  are  im- 
pressed upon  the  input  circuit  F  of  the  amplifying  bulbs  V-l,  the  out- 
put circuit  G  being  coupled  to  a  battery  of  power  bulbs  V-2  at  M-3.  The 
output  circuits  of  the  latter  are  inductively  coupled  to  the  antenna  at 
M-l.  So  far  the  circuit  does  not  differ  materially  from  that  described  in 


Vacuum  Tubes  In  Wireless  Communication 


147 


148 


Vacuum  Tubes  In  Wireless  Communication 


paragraph  56,  and  as  already  explained,  the  antenna  only  radiates  when 
the  transmitter  T  is  actuated. 

It  is  to  be  noted,  however,  that  through  the  transformer  M-4,  and 
the  telegraph  key  K-l,  currents  of  tlie  frequency  of  the  generator  can 
be  impressed  upon  the  input  circuit  F  of  the  amplifying  bulbs  F-l. 
Therefore,  during  the  moment  that  the  key  K-l  is  closed,  the  antenna 
will  radiate  at  the  frequency  of  the  alternator  F.  This  wave  motion  can 
be  detected  at  the  receiving  station  by  a  receiver  tuned  to  that  fre- 
quency. Thus,  the  current  of  the  carrier  wave  is  superposed  upon  the 
modulated  current  induced  in  the  antenna  circuit  by  the  microphone. 
Telegraphic  and  telephonic  signalling  may  then  be  carried  on  simul-, 
taneously.  During  the  time  that  the  key  K-\  is  closed,  the  speech  dis- 
tortion ordinarily  caused  by  the  presence  of  the  frequency  F  in  the 


Figure  98 

Receiving  apparatus  in  Englund's  duplex  system.    This  system  will 
receive  telegraphic  and  telephonic  signals  simultaneously. 

antenna  circuit  in  wireless  telephony,  is  experienced,  but  at  all  other 
times  the  frequency  of  the  carrier  wave  F  is  eliminated.  However,  it 
does  not  interfere  seriously  with  the  signals  of  speech  or  vocal  frequency. 

To  avoid  short-circuiting  the  amplifiers  F-l,  a  resistance  R  is  placed 
in  series  with  the  key  circuit. 

The  circuits  of  the  receiving  system  whereby  telegraphic  and  tele- 
phonic signals  may  be  recorded  at  the  wave  length  simultaneously  is 
shown  in  Figure  98. 

The  carrier  frequency  F  which  has  been  eliminated  at  the  trans- 
mitting station  for  wireless  telephony,  is  supplied  by  a  local  generator 
A-2  which  is  coupled  to  the  input  side  of  a  three-electrode  valve  V-3,  the 
output  circuit  of  which  is  connected  to  a  telephone  receiver  T-1, 


Vacuum  Tubes  In  Wireless  Communication 


149 


150  Vacuum  Tubes  In  Wireless  Communication 

The  secondary  of  the  receiving  transformer  M-3  is  shunted  by  an 
inductance  L-l  and  a  condenser  0-1,  the  circuit  further  containing  the 
inductances  L-2,  L-3  and  the  condensers  0-2  and  0-3.  This  circuit  will 
be  found  similar  to  B-l,  B-2  of  Figure  97,  performing  similar  functions. 
It  is  also  to  be  noted  that  the  input  side  of  a  vacuum  tube  VA  is  in- 
ductively coupled  to  L-l  by  transformer  MA.  It  is  in  this  circuit  that 
the  telegraphic  signals  are  detected. 

Keeping  in  mind  the  functions  of  the  branch  circuits  B-l  and  B-2, 
in  the  transmitter,  the  function  of  those  of  the  receiver  will  be  readily 
understood.  Thus,  oscillations  of  the  carrier  frequency  will  be  shunted 
through  L-l,  0-1.  Through  the  coupling  MA  they  are  impressed  upon 
the  input  circuit  of  the  tube  VA  and  detected  in  the  telephone  T-2.  Cur- 
rents of  the  carrier  frequency  cannot  appear  in  the  transformer  M-o 
which  serves  to  couple  the  antenna  system  to  the  input  circuit  of  the  valve 
V-3,  but  currents  of  modulated  frequency  are  readily  transformed 
through  i¥-5  because  of  its  tuning  and  thus  are  detected  in  the  tele- 
phone T-l. 

In  summary,  the  telegraphic  signals  are  detected  in  telephone  T-2  and 
telephonic  signals  in  T-l.  The  alternator  A-2  supplies  the  carrier  fre- 
quency F  which  has  been  eliminated  in  the  telephone  transmitter  circuits. 

Englund  has  disclosed  another  system  for  elimination  of  the  carrier 
frequency  at  the  transmitting  station  in  wireless  telephony,  it  being  a 
modification  of  the  circuits  disclosed  in  paragraph  56.  The  complete 
circuits  are  shown  in  Figure  99.  As  usual,  the  radio  frequency  carrier 
wave  is  generated  by  the  source  A-l  which  is  inductively  coupled  to  the 
input  side  of  the  three-electrode  tube  V-l  at  the  coupling  M,  the  same 
circuit  being  coupled  at  M-l  to  the  transmitter  circuit  including  the 
microphone  T,  and  the  battery!?.  The  output  circuit  of  V-l  is  inductively 
coupled  through  M-2  to  the  input  circuit  of  the  power  bulbs  V-2.  The 
output  circuit  of  the  latter  is,  in  turn,  inductively  coupled  to  the  antenna 
circuit  as  usual  at  MA. 

Up  to  this  point,  if  the  transmitter  T  be  spoken  into,  the  antenna 
would  radiate  at  three  frequencies,  that  is,  the  carrier  frequency  would 
not  be  eliminated.  A  special  balancing-out  circuit,  however,  is  provided, 
which  is  connected  to  the  alternator  A  A  in  the  following  way:  The 
input  side  of  a  vacuum  tube  V-3  is  coupled  to  the  alternator  through 
the  auto-transformer  M-3.  The  output  circuit  of  V-3  is  connected  to  a 
phase-regulating  device  P,  consisting  of  inductances,  capacity  and  resis- 
tance, as  shown.  This  circuit  is  in  turn  tapped  across  the  secondary  wind- 
ing of  the  transformer  M-2  so  that  currents  of  the  carrier  frequency  F 
which  may  be  induced  in  the  circuits  of  M-2  are  balanced  out  by  oppo- 
site phase  regulation.  The  antenna  then  radiates  only  during  the  produc- 
tion of  the  wave  of  vocal  frequency. 


PART  VIII 

THE    DYNATRON    DETECTOR    AND 
OSCILLATOR 

60.  THE  DYNATRON.— A  recent  development  in  the  design  of 
the  three-electrode  evacuated  tubes  is  the  so-called  dynatron  described  by 
Dr.  A.  W.  Hull  in  the  February,  1918,  issue  of  *"  Proceedings  of  the 
Institute  of  Radio  Engineers."  The  fundamental  construction  is  shown 
in  Figure  100.  The  tube  contains  a  filament  F,  a  perforated  anode  A, 
and  a  plate  P.  As  in  the  usual  tube  circuits,  the  filament  is  incandesced 

vwvww — 


R  N 

K  EXTERNAL 

CIRCUIT 

+ 

5-T  N 


B'2  iH  T- 

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Figure  100 

Showing    the    fundamental    construction    of    the 
dynatron. 


by  a  battery  JB-1,  and  the  anode  is  held  at  a  positive  potential  by  the 
battery  B-2. 

One  of  the  peculiar  operating  characteristics  of  the  dynatron  is  that 
within  a  certain  region  of  voltages  applied  to  the  plate  circuit,  the  device 


*The  dynatron  is  treated  at  length  in  Vol.  6,  No.  1,  "Proceedings  of  the  Insti- 
tute of  Radio  Engineers,"  from  which  the  material  of  this  chapter  has  been 
abstracted. 

151 


152 


Vacuum  Tubes  In  Wireless  Communication 


acts  as  a  simple  rectifier,  but  at  other  values  of  plate  voltage  it  acts  as 
a  true  negative  resistance,  namely,  an  applied  E.M.F.  will  set  up  a  cur- 
rent in  the  wrong  direction. 

When  the  filament  F  is  lit  to  incandescence  electrons  are  attracted 
to  the  anode  A  by  connecting  it  to  the  positive  pole  of  the  battery  B-2, 
but  some  of  them  pass  through  the  holes  in  the  anode  and  strike  the  plate 
P.  If  the  potential  difference  between  P  and  F  is  low,  the  electrons  will 
enter  the  plate  and  as  usual  set  up  a  negative  current  in  the  external 


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Figure  101 
Characteristic  curve  of  the  dynatron. 


circuit,  but  if  the  potential  of  P  is  raised  to  some  upper  value,  the  pri- 
mary electrons  will  strike  it  with  such  velocity  that  their  impact  will 
cause  the  emission  of  secondary  electrons  (from  the  plate). 

These  electrons  will  be  attracted  to  the  more  positive  anode  A.  The 
final  electron  current  received  by  the  plate  is  the  difference  between  the 
number  of  primary  electrons  that  strike  it  and  the  number  of  secondary 


Vacuum  Tubes  In  Wireless  Communication  153 

electrons  that  leave  it.  Dr.  Hull  states  that  the  number  of  primary  elec- 
trons depends  upon  the  temperature  of  the  filament,  but  they  are  inde- 
pendent of  the  voltage  of  the  plate.  The  number  of  secondary  electrons 
increases  rapidly  with  the  voltage  difference  between  the  plate  and  fila- 
ment and  may  exceed  the  number  of  primary  electrons.  In  fact,  each 
primary  electron  may  produce  as  many  as  20  secondary  electrons. 

The  phenomena  surrounding  the  functioning  of  the  dynatron  are 
shown  in  the  characteristic  curve  of  Figure  101.  The  horizontal  axis 
represents  the  voltage  of  the  plate  with  respect  to  the  negative  end  of 
the  filament ;  the  vertical  axis,  the  current  in  the  plate  circuit.  The  data 
for  the  plotting  is  secured  by  maintaining  the  filament  temperature  at 
some  constant  value  varying  the  voltage  of  the  plate  to  filament  by  mov- 
ing contact  T  up  tfhe  battery.  Contact  T-l  serves  to  vary  the  potential 
of  the  anode.  (The  anode  potential  when  properly  adjusted  remains 
fixed.) 

At  very  low  voltages,  as  is  clear  from  the  curve,  the  plate  current 
is  relatively  small,  but  at  about  25  volts  the  plate  receives  the  entire 
electronic  emission  of  the  filament.  For  higher  voltages  the  primary 
electronic  currents  remain  constant,  but  at  values  above  25  volts  the 
emission  of  secondary  electrons  enters  the  case ;  that  is,  the  primary  elec- 
trons strike  the  plate  with  such  velocity  as  to  cause  the  emission  of  sec- 
ondary electrons  which,  as  the  curve  indicates,  increase  rapidly  with  the 
voltage.  The  net  plate  current  therefore  decreases  as  may  be  seen  by  fol- 
lowing the  downward  slope  of  the  curve. 

At  approximately  100  volts  the  number  of  secondary  electrons  leav- 
ing the  plate  is  equal  to  the  number  of  primary  electrons  entering  it. 
The  plate  current  therefore  is  zero.  For  further  increases  of  voltage,  the 
secondary  electrons  exceed  the  primary  electrons,  that  is,  the  plate  ex- 
periences a  net  loss  of  electrons.  The  current  therefore  flows  in  the  oppo- 
site direction  to  the  impressed  voltage.  For  still  further  increases  of 
voltage,  say  200  volts,  a  point  is  reached  at  which  the  anode  is  no 
longer  sufficiently  positive  to  carry  away  all  the  secondary  electrons  from 
the  plate,  and  the  current  again  becomes  a  zero  and  then  rises  to  a  num- 
ber corresponding  to  the  number  of  primary  electrons. 

It  should  be  noted  from  the  curve  that  in  the  region  A0  to  C0,  that 
is  between  50  and  150  volts,  the  current  in  the  dynatron  decreases  linearly 
with  increase  of  the  voltage.  Here  the  dynatron  acts  as  a  true  nega- 
tive resistance.  For  example,  if  connection  is  made  from  the  tap  T  and 
the  plate  P,  the  dynatron  would  act  the  reverse  to  an  ordinary  or  positive 
resistance. 

To  make  the  dynatron  an  amplifier  we  insert  a  series  resistance  such 
as,  R,  Figure  100.  The  value  of  R  should  be  the  same  as  the  negative 


154 


Vacuum  Tubes  In  Wireless  Communication 


resistance  of  the  tube.  The  characteristic  curve  of  the  total  circuit  then 
becomes  that  shown  in  Figure  102.  It  is  clearly  seen  that  in  the  region 
of  the  plate  voltage  corresponding  to  E,  the  application  of  a  small  E.M.F. 
to  the  circuit  of  Figure  100  such  as  by  connection  to  the  terminals  N,  N, 
will  cause  a  very  large  change  in  the  total  current  through  the  circuit, 
the  amplification  only  being  limited  by  the  characteristic  curve  itself. 
It  is  pointed  out  that  while  the  total  resistance  of  the  circuit  of  Figure 


/ 


*£ 


E. 


M. 


z 


The  characteristic  curve 


Figure  102 
of  the  dynatron  for  voltage  amp1'  Nation. 


100  is  very  small,  the  resistance  of  its  parts  individually  is  not.  There- 
fore, a  small  change  in  applied  E.M.F.  will  cause  a  comparatively  large 
change  in  current  and  consequently  in  the  voltage  drop  across  each  part 
separately.  The  dynatron  thus  becomes  a  powerful  amplifier. 

61.  THE  DYNATRON  AS  AN  OSCILLATOR.—  The  plate  cir- 
cuit of  the  dynatron  if  connected  to  an  inductance-capacity  circuit  will 
oscillate  at  a  frequency  determined  largely  by  the  values  of  L  and  (7. 
Such  a  circuit  is  shown  in  Figure  103,  where  the  inductance  L  is  shunted 


Vacuum  Tubes  In  Wireless  Communication 


155 


by  the  condenser  C,  both  being  connected  in  series  with  the  circuit  of  the 
plate  P.  Since  there  is  but  one  oscillating  circuit  it  affords  greater  sim- 
plicity in  manipulation  than  the  usual  regenerative  systems.  Connected 
as  in  Figure  103  the  dynatron  may  be  employed  to  generate  the  local 
radio  frequency  currents  for  beat  reception  or  for  radio  frequency  meas- 
urements. Currents  at  frequencies  from  one-half  to  20,000,000  cycles 
per  second  are  readily  obtained. 


Figure  103 

The  connections  of  the  dynatron  for  the  production  of  radio  fre- 
quency currents. 


62.  THE  DYNATRON  AS  A  DETECTOR  OF  ELECTRICAL 
OSCILLATIONS. — If  connected  as  in  Figure  104,  the  dynatron  can  be 
employed  to  detect  electrical  oscillations  in  radio.  For  example,  if 
the  plate  potential  is  adjusted  by  moving  the  sliding  contact  T,  Figure 
100,  over  the  high  voltage  battery  B-2  we  arrive  at  the  points  C  or  A  in 
Figure  101.  The  radio  frequency  current  impressed  upon  the  plate  cir- 
cuit, which  in  this  case  includes  the  secondary  inductance  L-2,  the  shunt 
condenser  (7-2,  the  telephone  P-l  and  the  shunt  condenser  C-l,  will,  if  the 
dynatron  is  worked  near  either  point  A  or  C,  be  repeated  through  the 
telephone  with  distortion;  that  is,  there  will  be  obtained  a  radio  fre- 
quency component  of  the  plate  current  the  increase  of  which  exceeds  the 
decrease,  i.  e.,  a  rectification.  The  telephone,  therefore,  will  be  impulsed 
once  for  each  group  of  incoming  oscillations.  The  distributed  capacity 
of  the  telephone  winding  P-l  will  be  sufficient  to  pass  the  radio  frequency 
current  flowing  in  the  secondary  circuit,  but  owing  to  the  high  inductance 
of  the  telephone  windings  the  circuit  is  apt  to  oscillate  at  an  audio  fre- 


156 


Vacuum  Tubes  In  Wireless  Communication 


quency.  Audio  frequency  oscillation  may  be  prevented  by  shunting  the 
telephone  to  a  condenser  0-1  of  suitable  capacity. 

The  circuit  of  Figure  104  may  be  set  to  oscillate  at  a  frequency 
slightly  different  than  that  of  the  incoming  signals  thereby  producing 
beats.  Further  advantage  can  be  obtained  by  tuning  the  circuit  includ- 
ing the  telephone  P-l  and  the  condenser  (7-1  to  the  desired  audio  fre- 
quency. This  is  accomplished  by  adjusting  the  capacity  of  C-l  and  the 
negative  resistance  of  the  tube  to  neutralize  the  resistance  of  the  tele- 
phone P-l  to  a  particular  audio  or  group  frequency.  If  the  frequency  of 
the  circuit  coincides  with  the  group  frequency  of  the  incoming  oscilla- 
tions, the  sensitiveness  of  the  system  becomes  very  great. 

In  a  circuit  of  this  kind,  the  energy  consumed  in  the  detector  does 
not  decrease  the  amplification  because  the  dynatron  can  be  adjusted 
just  to  neutralize  this  loss.  The  selectivity  is  therefore  relatively  great, 
for  the  detector  absorbs  no  energy  as  in  circuits  heretofore;  i.  e.,  the 
damping  is  not  increased  by  the  detector. 


C-i 


Figure  104 
Connections  of  the  dynatron  as  an  oscillation  detector  in  radio. 


63.  THE  DYNATRON  AS  A  COMPENSATOR  FOR  CIR- 
CUIT LOSSES.— Two  circuits  disclosed  by  Dr.  Hull  show  how  the 
dynatron  may  be  employed  to  supply  the  energy  losses  in  any  oscillation 
circuit.  The  circuit  therefore  behaves  as  regards  selectivity,  damping 
and  sensitiveness,  to  an  impressed  radio  frequency  current  like  a  circuit 
having  zero  resistance,  that  is,  the  dynatron  feeds  back  to  the  circuit 
that  energy  which  ordinarily  is  lost  by  "positive"  resistance.  In  the 
diagram  of  Figure  105,  the  dynatron  is  connected  in  the  plate  circuit 
of  a  three-electrode  tube  for  aperiodic  amplification.  The  filament  of  the 
three-electrode  tube  is  shown  at  F,  the  grid  at  G,  and  the  plate  at  P.  The 
dynatron  is  indicated  by  the  filament  F',  the  anode  A',  and  the  plate  P'. 


Vacuum  Tubes  In  Wireless  Communication 


157 


The  filament  F  is  incandesced  by  the  battery  J5-1,  the  filament  F'  by  the 
battery  #-2,  and  the  anode  of  the  dynatron  is  held  at  a  positive  potential 
by  the  battery  B-3.  Battery  5-4  is  that  normally  of  the  plate  circuit  P, 
but  it  includes  the  resistance  E  and  the  current  translator  P-l,  R  being 
adjusted  just  to  neutralize  the  negative  resistance  of  the. dynatron  F',  A', 
P'.  The  current  through  the  pliotron  and  for  constant  grid  voltage  in 
creases  with  increasing  voltage  of  the  plate,  that  is,  the  circuit  has 
the  characteristics  of  a  positive  resistance  which  limits  its  amplifying 
power,  but  this  may  be  neutralized  by  connecting  the  dynatron  as  shown 
in  Figure  105. 


VALVE 


DYNATRON 


VOLTAGE 
TO  BE 
AMPLIFIED 


T 


Figure  105 

The  dynatron  connected  to  compensate  for  the  losses  in  the  piate  cir- 
cuit of  a  three-electrode  vacuum  tube. 

Using  a  pliotron  of  100,000  ohms  resistance  and  a  series  resistance, 
R,  of  250,000  ohms,  Dr.  Hull  states  that  the  D.  C.  voltage  amplification 
was  increased  from  12  fold  for  the  pliotron  to  625  fold  for  the  circuit 
of  Figure  105. 

The  dynatron  connected  to  compensate  the  grid  circuit  losses  of  a 
pliotron  is  shown  in  Figure  106.  An  increase  of  the  voltage  of  the  grid 


DYNATRON 


Figure  106 

The  dynatron  connected  to  compensate  for  the  losses  in  the  grid 
circuit  of  a  three-electrode  vacuum  tube. 


158  Vacuum  Tubes  In  Wireless  Communication 

of  the  pliotron  detector  is  opposed  by  a  leakage  current  which  increases 
with  the  voltage  as  in  a  positive  resistance,  and  also  by  the  counter 
E.M.F.  and  losses  in  its  own  and  coupled  antenna  circuit.  These 
losses  may  be  neutralized  by  connecting  a  dynatron  in  parallel  with  the 
grid  circuit  of  the  pliotron  as  in  Figure  106.  It  is  remarked  that  with 
this  arrangement  the  intensity  of  weak  signals  from  a  spark  set  was  in- 
creased from  audibility  to  a  roar. 


Figure  107 
The  pliodynatron. 

\ 

64.  THE  PLIODYNATRON.— As  in  the  usual  three-electrode 
tube,  the  electrostatic  field  of  a  grid  may  be  employed  to  control  the 
number  of  electrons  reaching  the  plate  in  a  dynatron.  A  tube  con- 
structed in  this  way,  shown  in  Figure  107,  is  called  a  pliodynatron.  Its 
construction  is  in  all  respects  similar  to  the  simple  dynatron  with  the 
exception  of  the  grid  surrounding  the  filament. 

The  negative  resistance  of  the  pliodynatron  makes  it  a  powerful 
amplifier.  In  the  simple  pliotron  an  increase  of  grid  potential  by  in- 
creasing the  current  through  the  load  in  the  plate  circuit  and  therefore 
the  voltage  drop  over  the  load  lowers  the  voltage  of  the  plate  and  this 
tends  to  decrease  the  plate  current  and  to  oppose  the  effect  of  the  grid.  In 
the  pliodynatron,  a  decrease  in  the  plate  voltage  means  an  increase  in 


Vacuum  Tubes  In  Wireless  Communication 


159 


current  which  may  be  very  large  if  the  positive  and  negative  resistances 
are  adjusted  equally. 

65.  THE  PLIODYNATRON  AS  A  DETECTOR.— The  circuit 
for  the  pliodynatron  as  a  detector  of  radio  frequency  currents  is  shown 
in  Figure  108,  the  primary  winding  of  the  receiving  tuner  being  indi- 
cated at  L-l,  the  secondary  winding  at  L-2,  the  shunt  condenser  at  0-1, 
the  grid  element  at  G,  the  filament  at  F,  the  anode  at  A,  and  the  plate 
at  P.  The  filament  is  incandesced  by  B-l  and  the  anode  is  excited  by  the 
battery  B-2.  The  potential  of  the  plate  P  in  respect  to  the  anode  is  regu- 
lated by  the  variable  contact  T  at  the  battery  B-2.  The  plate  circuit  in- 


Figure  108 

The  connections  of  the  pliodynatron  as  a  detector  of  radio  frequency 
currents. 

eludes  the  inductance  L-3,  a  shunt  condenser  0-2  and  the  telephone  con- 
denser 0-3.  The  function  of  the  circuit  L-3,  0-2,  is  to  increase  the  selec- 
tivity of  the  circuit  or  to  amplify  the  incoming  signal.  Thus  the  cir- 
cuit jC-3,  0-2,  may  be  set  on  the  verge  of  oscillation  for  the  reception  of 
damped  waves,  or  set  into  oscillation  at  a  slightly  different  frequency 
than  that  of  the  incoming  signal,  for  the  reception  of  undamped  waves. 
Used  in  the  latter  way,  beats  will  occur  in  the  plate  circuit.  The  circuit 
P-l,  0-3,  can  be  tuned  to  the  beat  frequency  for  greater  selectivity.  This 
circuit  can  be  brought  to  the  verge  of  oscillation  at  an  audio  frequency  by 
adjusting  the  negative  resistance.  The  final  adjustment  for  radio  fre- 
quency tuning  is  made  by  varying  the  ratio  of  L-3  to  0-2,  keeping  their 
product  constant  for  any  particular  applied  frequency. 

66.    THE  PLIODYNATRON  IN  RADIO  TELEPHONY.— The 

pliodynatron  may  be  employed  to  generate  the  required  radio  frequency 
current  for  radio  telegraphic  or  telephonic  transmission,  as  has  already 
been  shown  in  Figure  103.  The  circuit  for  radio  telephony  is  shown  in 
Figure  109.  The  primary  circuit  L-l,  0,  is  inductively  coupled  to  the 


160 


Vacuum  Tubes  In  Wireless  Communication 


antenna  through  coils  L}  L-l.  Owing  to  the  instability  of  a  circuit  pos- 
sessing negative  resistance,  oscillations  will  occur  in  the  circuit  L-l,  C,  at 
a  frequency  varying  as  their  product.  The  amplitude  of  these  oscilla- 
tions may  be  controlled  by  the  grid  G,  which  is  connected  to  one  leg  of 
the  secondary  winding  of  a  speech  frequency  transformer.  The  other 
leg  of  this  transformer  is  connected  to  one  terminal  of  the  plate  battery 


Figure  109 
The  connections  of  the  pliodynatron  in  wireless  telephony. 


B-2.  A  microphone  M  and  a  battery  BA  cause  speech  frequency  varia- 
tions of  the  potential  of  the  grid  to  the  filament  and  the  amplitude  of  the 
energy  radiated  from  the  antenna  is  varied  in  accordance. 

Dr.  Hull  reports  that  with  a  small  tube  giving  about  ten  watts  it 
became  possible  to  telephone  by  wireless  sixteen  miles  with  good  inten- 
sity and  articulation. 


APPENDIX 

Diagrams  Illustrating  Certain  Fundamental  Actions  of  the 

Vacuum  Tube  as  an  Oscillation  Detector 

In  Radio  Telegraphy 


Figure  1 


The  filament  F  when  heated  by  battery  B-l  emits  electrons  which  are 
drawn  to  plate  P  when  it  is  connected  to  the  positive  pole  of  a  high  voltage 
battery  such  as  B-2.  Current  from  battery  B-2  then  flows  from  P  to  the  nega- 
tive side  of  F  through  the  telephones  back  to  the  negative  terminal  of  the 
battery.  (It  is  to  be  noted  that  the  assumed  direction  of  the  current  is  opposite 
to  the  flow  of  electrons.) 

.      161 


162 


Vacuum  Tubes  In  Wireless  Communication 


Figure  2 

If  filament  F  and  plate  P  are  connected  to  the  terminals  of  a  coil  L-l  in 
which  is  induced  an  alternating  E.M.F.  the  valve  is  conductive  in  the  direction 
from  P  to  F  but  not  in  the  opposite  way.  Hence  the  alternating  current  in- 
duced in  coil  L-l  is  rectified  and  the  telephone  P-l  is  traversed  by  uni-direc- 
tional  currents. 


Figure  3 


If  a  metallic  element  called  the  grid,  G,  is  inserted  between  the  filament 
and  plate  it  captures  a  certain  number  of  electrons  and  its  potential  is  lowered 
thereby  reducing  the  flow  of  current  from  P  to  F.  In  other  words,  the  flow  of 
current  from  B-2  is  reduced. 


Vacuum  Tubes  In  Wireless  Communication 


163 


P-2 


Figure  4 

If  the  grid,  G,  is  charged  to  a  sufficiently  high  negative  potential  relative 
to  the  filament  by  an  external  E.M.F.  such  as  may  be  supplied  by  a  battery  B-3, 
the  flow  of  electrons  from  the  plate  is  completely  obstructed.  The  circuit  of 
the  battery  B-2  from  P  to  F  is  then  opened. 


Figure  5 


If  the  grid  G  is  charged  to  a  positive  potential  by  an  external  E.M.F.  nor- 
mal flow  of  electrons  to  the  plate  takes  place  and  the  circuit  of  battery  B-2  is 
restored. 


164 


Vacuum  Tubes  In  Wireless  Communication 


L-3 


Figure  6 

If  the  terminals  of  a  coil  such  as  L-l  are  connected  to  the  grid,  G,  and 
the  filament,  F,  and  an  alternating  E.M.F.  of  radio  or  audio  frequency  is 
induced  therein,  the  negative  alternation  reduces  the  flow  of  electrons  between 
F  and  P-and  the  positive  alternation  increases  the  flow  of  electrons  between 
F  and  P.  The  plate  current  therefore  rises  and  falls  at  the  frequency  of  the 
E.M.F.  impressed  upon  the  grid  circuit.  An  alternating  current  of  increased 
power  can  then  be  withdrawn  from  the  terminals  C,  D,  of  the  coil  L-3.  The 
additional  energy  for  amplification  is  supplied  by  the  source  B-2.  Within  a 
certain  range  of  applied  E.M.F's.  the  current  impressed  upon  the  grid  circuit 
will  be  repeated  in  the  plate  circuit  B-2,  L-2  without  distortion.  On  the  other 
hand,  under  certain  adjustments  of  filament  temperature  and  plate  voltage,  what 
amounts  to  a  rectified  current  will  flow  through  the  coil  L-2  when  an  alternat- 
ing E.M.F.  is  impressed  upon  the  filament  F  and  the  grid  G;  that  is,  the  posi- 
tive halves  of  the  repeated  plate  currents  may  exceed  the  negative  halves  or 
vice  versa. 


Figure  7 

By  inserting  a  grid  battery  B-3,  shunted  by  the  potentiometer,  P-2,  in 
the  grid  circuit  of  the  valve  F,  G,  the  potential  of  the  grid  in  respect  to 
the  filament  can  be  maintained  so  that  the  negative  half  of  an  impressed  al- 
ternating E.M.F.  causes  a  very  slight  reduction  of  the  plate  current  and  the 
positive  half  a  great  increase  of  the  plate  current.  This  effect  is  obtained 
when  the  tube  is  worked  at  the  lower  bend  of  the  characteristic  curve.  At 
the  upper  bend,  the  reverse  effect  takes  place,  that  is,  the  negative  half  of  an 
impressed  alternating  E.M.F.  decreases  the  plate  current  greatly,  and  the  posi- 
tive half  increases  it  slightly.  In  either  case,  what  amounts  to  a  rectified  cur- 
rent flows  in  the  plate  circuit  P,  B-2,  L-2,  P-l,  F.  The  same  effect  is  obtained 
from  the  circuit  in  Figure  6,  but  with  the  potentiometer  and  battery  of  Figure  7 
it  is  less  difficult  for  the  operator  to  obtain  the  desired  operating  charac- 
teristic. 


Vacuum  Tubes  In  Wireless  Communication 


165 


B-2 


P-l 


Figure  8 


By  connecting  a  condenser  C  in  series  with  the  grid,  groups  of  radio  fre- 
quency oscillations  such  as  A,  B,  C  are  rectified  between  G  and  F,  the  con- 
denser C  accumulating  a  charge  which  is  negative  on  the  grid  side  of  C.  This 
increased  negative  potential  reduces  the  electron  current  from  filament  to  plate, 
that  is,  the  flow  of  current  from  battery  B-2  is  reduced.  At  the  termination  of  a 
group  of  grid  oscillations  the  charge  leaks  out  the  condenser  C,  either  through 
the  valve  or  through  a  special  leak  resistance.  The  grid  then  returns  to  normal 
potential,  and  the  plate  current  returns  to  normal  value.  The  telephone  P-l  will 
then  be  impulsed  once  for  each  group  of  radio  frequency  oscillations.  During 
the  time  that  the  radio  frequency  oscillations  undergo  rectification,  the  radio 
frequency  current  impressed  upon  the  grid  circuit  is  repeated  in  the  plate  cir- 
cuit, but  this  current  is  not  heard  in  the  telephone  P-l  because  its  frequency 
is  above  audition. 


166 


Vacuum  Tubes  In  Wireless  Communication 


L-i 


(JQOOOIK 


Figure  9 


By  coupling  the  plate  and  grid  circuits  of  the  vacuum  tube  through  a 
transformer  such  as  P',  S  either  the  radio  or  audio  frequency  variation  of  the 
continuous  plate  current  (such  as  is  obtained  during  the  reception  of  wireless 
signals)  can  be  impressed  upon  the  grid  and  re-enforced;  that  is,  part  of  the 
energy  in  the  plate  circuit  is  fed  back  to  the  grid  circuit  for  amplification.  If 
it  is  desired  to  amplify  radio  frequencies,  transformers  P',  S,  is  an  air  core 
radio  frequency  transformer,  but  for  the  amplification  of  audio  frequencies, 
windings  P'  and  S  may  have  a  henry  or  more  inductance  and  be  provided  with 
an  iron  core.  A  circuit  of  this  kind  is  termed  a  regenerative  system.  Incom- 
ing radio  signals  may  thus  be  enormously  amplified. 

If  the  coupling  of  P'  to  S  is  sufficiently  close,  the  valve  will  be  set  into 
self-oscillation  at  a  radio  or  an  audio  frequency.  The  tube  thus  becomes  a  gen- 
erator of  sustained  oscillations  which  may  be  employed  for  transmission  of 
radio  telephonic  or  telegraphic  signals,  or  for  the  reception  of  continuous  waves 
by  the  phenomenon  of  beats  or  otherwise. 


Vacuum  Tubes  In  Wireless  Communication 


167 


B-5 


Figure   10 


If  groups  of  radio  frequency  oscillations,  such  as  A,  B,  C,  are  impressed 
upon  the  coil  L-l,  the  plate  circuit  including  the  coil  L-2  is  traversed  by  an 
audio  and  a  radio  frequency  current.  Either  the  audio  or  radio  frequency 
component  of  the  continuous  plate  current  can  be  amplified  by  coupling  L-2 
to  the  grid  circuit  of  a  second  valve  F',  G'  P'  through  coil  L-3.  If  the  audio 
frequency  component  is  to  be  amplified,  L-2,  L-3  is  an  iron  core  transformer 
of  audio  frequency  dimensions;  but  if  it  is  desired  to  amplify  the  radio  fre- 
quency component  of  the  continuous  plate  current,  transformer  L-2,  L-3  is  of 
the  air  core  type  and  of  radio  frequency  dimensions.  Several  tubes  may  be 
connected  in  this  way  for  further  amplification.  Cascade  amplifiers  are  used 
in  wire  telephony  for  the  amplification  of  long-distance  telephone  signals  as 
well  as  in  radio.  In  fact,  the  tubes  may  be  employed  to  amplify  the  input  of 
any  variable  wave  form. 


REVIEW  QUESTIONS 


INTRODUCTION 


Ques.  (1)  State  the  distinction  drawn  between  audio  and  radio  frequency 
currents. 

Ques.  (2)  How  may  radio  frequency  currents  be  made  audible  in  a  telephone 
receiver? 

Ques.  (3)  State  three  methods  by  which  continuous  oscillations  may  be  made 
audible  in  a  telephone. 

Ques.  (4)  If  a  single  group  of  radio  frequency  oscillations  are  rectified  by  an 
oscillation  detector,  what  is  the  effect  upon  the  telephone 
diaphragm? 

Ques.  (5)  What  are  the  two  fundamental  circuits  of  the  inductively  coupled 
receiving  tuner? 

Ques.  (6)  How  is  the  receiver  transformer  and  associated  tuning  apparatus 
adjusted  to  establish  resonance  with  the  distant  transmitter? 

Ques.  (7)  Over  what  range  of  frequencies  will  the  telephone  receiver  give 
maximum  response  with  a  minimum  of  current? 


PART  I 

Ques.  (1)  What  is  the  direction  of  the  electron  current  within  the  two- 
electrode  tube? 

Ques.  (2)  Explain  by  diagram  a  simple  test  by  which  the  rectifying  properties 
of  the  tube  can  be  demonstrated. 

Ques.  (3)  Explain  what  is  meant  by  the  non-uniform  conductivity  of  the 
vacuum  tube;  how  can  advantage  be  taken  of  the  tube's  non- 
uniform  conductivity  in  the  reception  of  radio  signals? 

Ques.  (4)  Explain  the  phenomena  of  the  tube  which  cause  it  to  reach  the 
point  of  saturation. 

169 


170  Vacuum  Tubes  In  Wireless  Communication 


PART  II 

Ques.  (1)  Show  by  diagram  three  practical  circuits  for  the  two-electrode  valve 
as  an  oscillation  detector  in  radio. 

Ques.  (2)  What  is  the  effect  of  inserting  the  grid  element  in  a  two-electrode 
vacuum  tube? 

Ques.  (3)  If  the  grid  is  charged  to  a  high  negative  potential  by  an  external 
E.M.F.,  how  does  it  affect  the  electron  current;  similarly  what 
is  the  effect  if  the  grid  is  charged  to  a  positive  potential? 

Ques.  (4)  If  an  alternating  E.M.F.  be  impressed  upon  the  grid  and  filament  of 
the  three-electrode  tube,  state  what  occurs  in  the  plate  circuit? 

Ques.  (5)  Explain  the  functioning  of  the  three-electrode  valve  as  an  oscilla- 
tion detector  with  a  grid  condenser. 

Ques.  (6)  Explain  the  action  of  the  three-electrode  tube  as  an  oscillation  de- 
tector without  the  grid  condenser. 

Ques.  (7)  What  adjustments  are  necessary  in  order  that  the  three-electrode 
tube  may  be  employed  as  a  distortionless  repeater? 

Ques.  (8)  How  can  the  three-electrode  tube  be  adjusted  to  set  up  a  rectified 
current  in  its  plate  circuit? 

Ques.  (9)  What  is  the  effect  of  tuning  the  plate  circuit  of  the  three-electrode 
tube? 

Ques.  (10)  Explain  what  is  meant  by  the  "input"  and  "output"  circuits  of  the 
vacuum  tube. 


PART  III 

Ques.  (1)  Explain  what  is  meant  by  a  cascade  connection  of  vacuum  tubes. 

Ques.  (2)  What  are  the  principal  points  of  difference  between  the  cascade 
radio  frequency  amplifier  and  the  cascade  audio  frequency 
amplifier? 

Ques.  (3)  Which  of  the  two  cascade  systems  (radio  or  audio  frequency)  gives 
the  greatest  selectivity? 


PART  IV 

Ques.  (1)  What  is  the   effect  of  regenerative   coupling  in  the  vacuum  tube 
system?  ' 

Ques.  (2)  Explain  how  the  radio  frequency  regenerative  amplifier  is  adjusted 
to  amplify  damped  oscillations? 


Vacuum  Tubes  In  Wireless  Communication  171 

Ques.  (3)  What  are  the  differences  between  the  audio  frequency  regenerative 
amplifier  and  the  radio  frequency  regenerative  amplifier? 

Ques.  (4)  Show  by  diagram  a  circuit  employing  electrostatic  regenerative 
coupling. 

Ques.  (5)  If  during  the  reception  of  radio  signals  the  incoming  currents  are 
repeated  without  distortion  in  the  plate  circuit,  how  can  the 
repeated  plate  currents  be  made  audible  in  the  telephone? 


PART  V 

Ques.  (1)  What  are  the  advantages  of  a  combined  regenerative  and  cascade 
amplification  system? 

Ques.  (2)  For  selectivity,  which  is  preferable,  the  audio  frequency  regenerative 
cascade  system  or  the  radio  frequency  regenerative  cascade 
system? 


PART  VI 

Ques.  (1)  State  three  methods  by  which  continuous  oscillations  may  be  made 
audible  in  the  head  telephone. 

Ques.  (2)  Explain  the  fundamental  theory  of  the  beat  receiver. 

Ques.  (3)  What  are  the  advantages  of  the  beat  receiver  over  the  tikker? 

Ques.  (4)  Show   by   diagram   and   explain   the   functioning   of   the    external- 
heterodyne  receiver  and  the  self-heterodyne  receiver. 

Ques.  (5)  Explain  how  the  circuits  of  the  three-electrode  tube  can  be  set  into 
oscillation  at  a  radio  frequency. 

Ques.  (6)  Show  the  circuits  of  an  audio  frequency  tuning  system. 
Ques.  (7)  What  are  the  advantages  of  an  audio  frequency  tuner? 

Ques.  (8)  Show  by  diagram  how  continuous  waves  may  be  detected  by  the 
vacuum  tube  without  employing  the  beat  phenomenon. 

Ques.  (9)  What  are  the  advantages  of  a  cascade  regenerative  system  for  beat 
reception? 

Ques.  (10)  Why   is   amplification   obtained   through   the  use   of  open  circuit 
oscillators? 

Ques.  (11)  Show  by  diagram  how  the  three-electrode  tube  may  be  connected 
up  for  the  generation  of  radio  or  audio  frequency  currents. 


172  Vacuum  Tubes  In  Wireless  Communication 

PART  VII 

Ques.  (1)  Explain  three  methods  by  which  the  antenna  currents  of  a  radio 
telephone  transmitter  may  be  modulated  at  a  vocal  frequency. 

Ques.  (2)  In  what  part  of  the  circuits  of  the  three-electrode  vacuum  tube  is 
the  connection  of  the  microphone  transmitter  most  effective  for 
modulation? 

Ques.  (3)  Show  by  diagram  how  the  vacuum  tube  may  be  connected  up  for 
the  production  of  radio  frequency  oscillations  and  include  a 
battery  of  tubes  for  the  amplification  of  their  output. 

Ques.  (4)  Show  by  diagram  how  the  carrier  wave  in  radio  telephony  can  be 
eliminated  except  at  such  times  as  the  microphone  is  actuated. 

PART  VIII 

Ques.  (1)  What  are  the  fundamental  points  of  difference  between  the  con- 
struction of  the  dynatron  and  the  three-electrode  vacuum  tube? 

Ques.  (2)  Show  the  circuits  of  the  dynatron  for  use  as  a  detector  of  electrical 
oscillation. 

Ques.  (3)  Show  the  circuits  of  the  pliodynatron  as  an  oscillation  detector  in 
radio.  » 

Ques.  (4)  Explain  the  operation  of  the  dynatron  as  an  oscillation  detector 
from  its  characteristic  curve. 


INDEX 


Page 

A  MPLIFIER 
"•    cascade  audio  frequency. 57,  58,  159 

cascade  radio  frequency 51,  52 

combined  radio  and  audio  fre- 
quency cascade 61 

curves  of  cascade 55,  56 

regenerative  vacuum  valve.. 62,  76 

Armstrong,  Capt.  E.  H 19 

Audio   frequency   tuning. .  .100,  101,  102 
Audio  and  radio  frequency  currents 

definition  of   1 

T3EAT    CURRENTS 5 

13   Beat  Receiver 

curves  of  90,  91 

phenomena  of 86,  87,  88 

regenerative  type  of 97,  98 

theory  of 84,  85,  86 

Beat  Reception  from  damped  wave 

transmitters    113 

(CASCADE   AMPLIFICATION..  108 

^*  Coils,  multilayered  type  of 12 

Continuous  oscillations,  detection  of      5 
Continuous  waves 

reception  of 82,  83,  84 

reception  of,  without  beats .  103,  107 
reception    of,    by    rotary    con- 
denser     114,  115 

DETECTOR,  OSCILLATION 
coherer    4 

dynatron    , 156 

Fleming    21 

Marconi    magnetic 4 

pliodynatron 159 

slipping  contact  type  of 5 

three-electrode     37 

two-electrode    22 

Dynatron 

as  an  amplifier    154 

as  an  oscillator   155 

as  a  detector    155,  156 

as    a    compensator    of    energy 
losses 157,  158,  159 


Page 

PLECTRON 

*-*     definition  of 20,  21 


PLEMING,  DR.  j.  A 19 

Fleming  Oscillation  Valve.... 21,  37 

action  of  simple 29 

non-uniform  conductivity  of..  .     24 
practical  circuits  for.. 33,  34,  35,  36 

rectifying  properties  of 23 

saturation  characteristic  of 26 

space  charge  of . . . ., 28 

temperature  limitation  of 27,  28 

Franklin's  "Reaction"  Circuits 73,  74 


HETERODYNE  RECEIVER 
amplification  by 119,  120,  121 

selectivity  by  the 91,  92 

simple  type  of 88,  89 

with   vacuum   valve,  as   detec- 
tor   92,  93 


T  NDUCTANCE 

multipoint  tuning 13 

Ions,  positive  and  negative 20 


TTENOTRON    122,123 

JV     connections  for  126 

T   ANGUMUIR,  DR.  IRVING. ...     19 


OSCILLATIONS,  ELECTRICAL 

^    damped  and  undamped 2 

detection  of  continuous 5 

Oscillators,  open  circuit. ..  .116,  117,  118 


pLATE  CURRENT 
*-       radio  and  audio  frequency  com- 
ponent of 48 


173 


174 


Vacuum  Tubes  In  Wireless  Communication 


Page 

Pliotron 123,  124,  125 

as  generator  of  high  voltage  at 

radio  frequencies  127,  128 

as  generator  of  heavy  currents 

at  radio  frequencies 125,  126 

connections  for 126 

Pliodynatron 

as  a  detector 159 

in  radio  telephony 159,  160 

T>  ECEIVER 

AV    modified    Weagant    undamped 

wave 103,  104 

telephone,  phenomena  of 

1,  15,  16,  17 
Weagant  undamped  wave..  102,  103 

Receiver  Circuits 

conductive  type  of 10 

electrostatically  coupled  type  of     10 

inductive  type  of 9 

open  oscillator  type  of 11 

Receiver  Telephone 15 

action  of  16,  17 

Regenerative  Amplifier 
Armstrong's 

63,  64,  65,  66,  67,  68,  69,  70,  71,  72 

audio  frequency 69 

combined  audio  and  radio  fre- 
quency       70 

electrostatic    and    direct    mag- 
netic coupling  for 70,  71 

simple  type  of 75,  76 

ultra-audion  72 

Regenerative  Cascade  Systems .  .77,  78,  79 
audio  frequency 80 

Regenerative    Cascade    Systems    for 

beat  reception....  109,  110,  111,  112 

Rectifiers 

types  of 3 

use  of  3 

SELECTIVITY   IN   RADIO   RE- 

0      CEIVERS    14 

Self-heterodyne 

phenomena  of  the 99 


necessity  for  .................     11 

Tuned  Plate  Circuit  ..............  49,  50 

Armstrong's    ............  66,  67,  68 

Transmitter 

radio  frequency  circuits  of  ____     12 


,  THREE-ELECTRODE 
characteristic  curve  of 

38,  39,  43,  44,  48,  49 

as  an  electron  relay 41 

as  generator  of  radio  frequency 

oscillations   95 

apparatus   for    obtaining   char- 
acteristic curve  of 40 

fundamental  construction  of. ..     37 
modified    open   circuit,    oscilla-. 

tors  for  118 

open  circuit,  oscillators  for.  116,  117 

rectification  by 47 

relaying  action  of 44 

terminology   of 41 

use  of,  with  grid  condenser..       45 


WAVE    SYSTEMS 

distinction  between  continuous 
and  discontinuous 

Weagant,  Roy  A 19 

Wireless  Telephony 

Englunds'  system  for 

135,  136,  137,  138,  146 
147,  148,  149,  150 
Espenschied's     duplex     system 

for 140,  145 

Carson's  system  for 139 

Hund's  system  for 134,  135 

Simple    circuits    of    three-elec- 
trode tube  for..  131,  132,  133,  134 


"Y»   CIRCUIT  RECEIVER 
-^          R.  Weagant  ....102,  103,  104 


MEMORANDA 


MEMORANDA 


MEMORANDA 


MEMORANDA 


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PRACTICAL    WIRELESS    TELEGRAPHY 

By  ELMER  E.  BUCHER 

Instructing  Engineer  of  the  Marconi  Company 

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At  Last — a  Practical  Book  tor  Aviators 

Practical  Aviation  for  Military  Airmen 

By  MAJOR  J.  ANDREW  WHITE 

Author    of   "Military    Signal    Corps    Manual." 
Chief    Signal    Officer,    American    Guard. 

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Principles  of  flight;  construction  of  the  airplane;  rigging;  directions  for  first  flights, 
cross-country  and  night  flying ;  construction,  operation  and  care  of  engines ;  reconnais- 
sance, map  reading,  signaling  and  co-operation  with  military  bodies  ;  radio  and  its  uses  ; 
machine  gunnery  and  bombing  from  airplanes,  etc. 

Cloth,  Price  $1.50  Ready  About  September  First 

How  to  Pass  U.  S.  Government  Wireless 
License  Examinations 

By  E.  E.  BUCHER 

New  Edition  Largely  Revised  and  Extended.     142  Questions  and  Answers 
An  ideal  quiz  book  when  used  with  PRACTICAL  WIRELESS  TELEGRAPHY 

CONTENTS 

Explanation  of  Electrical  Symbols — Definitions  of  Electrical  Terms — Part  I.  Transmitting 
Apparatus — Part  II.  Motor  Generators — Part  III.  Storage  Batteries  and  the  Auxiliary 
Set — Part  IV.  Antenna  of  Aerials — Part  V.  Receiving  Apparatus — Part  VI.  Radio 
Laws  and  Regulations — Part  VII.  General  Information  Concerning  Operator's  License 
Examinations — Practical  Equations  for  Radio  Telegraphy — Equations  for  Ordinary  Power 
Work-  PRICE  50c.  Net 

Elementary    Principles    of  Wireless    Telegraphy 

By  R.  D.  BANGAY 

Since  the  book  has  been  used  largely  in  the  training  of  Telegraphists  who  are  frequently 
called  upon  to  take  sole  charge  of  complete  Wireless  Telegraph  installations,  the  author 
has  endeavored  to  cover  all  parts  of  the  transmitting  and  receiving  apparatus  in  such 
a  way  as  to  give  the  student  a  sound  working  knowledge  of  the  apparatus  entrusted 
to  his  care. 

CONTENTS 

Continental  Code — Symbols  Used  in  Diagrams — Electricity  and  Magnetism — Magnetism — 
The  Principles  of  Wave  Motion — Pressure  Waves — Production  of  Waves — Production  of 
High-Frequency  Oscillations — Production  of  Oscillating  Currents  in  an  Aerial — Coupled 
Oscillatory  Circuits — The  Wavemeter — Wireless  Telegraph  Receivers — The  Tuning  Buzzer 
— The  Electrolytic  Detector — The  Magnetic  Detector — "Atmospherics" — Aerials — Distribu- 
tion of  Potential  and  Current  Along  Aerials — Masts — The  Insulation  of  Aerials — INDEX. 

Cloth,  PRICE  $1.00 

Radio  Instruments  and  Measurements 

A  valuable   reference  book  for  every  day   use. 

Every  instructor  and  teacher  of  radio,  whether  teaching  in  college  or  Government  training 
school  will  find  this  book  a  valuable  addition  to  his  radio  library. 

CONTENTS 

Part  I.     Theoretical  basis  of  radio  measurements.     The  fundamentals  of  electromagnetism. 

The  principles  of  alternating  currents.     Radio  circuits.     Damping. 
Part    II.     Instruments   and   methods   of    radio  measurement.      Wave   meters.      Condensers. 

Coils.     Current    measurement.      Resistance   measurement.      Sources    of    high-frequency 

current. 
Part    III.     Formulas    and    Data.      Calculation    of    capacity.      Calculation    of    inductance. 

Design   of  inductance  coils.      High-frequency   resistance.     Miscellaneous   formulas   and 

data-  In  Press-Ready   September  first 


Specialists  in 
Wireless  Literature 


WIRELESS  PRESS,  Inc. 


RADIO  TELEPHONY 

By  Alfred  N.  Goldsmith,  Ph.  D. 

Fellow  of  the  Institute  of  Radio  Engineers 

Member  of  the  American  Institute  of  Electrical  Engineers 

Director  of  the  Radio  Telegraphic  and  Telephonic  Laboratory 

of  the  College  of  the  City  of  New  York 

This  complete  text  on  radio  telephony  is  intended  for  radio  engineers, 
radio  electricians  in  the  Navy,  men  in  the  Signal  Corps  and  especially 
men  in  the  Aviation  Service  who  handle  radio  equipment.  Amateurs  and 
others  who  desire  to  be  clearly  informed  concerning  this  newest  and  most 
interesting  branch  of  electric  communication  will  want  this  book. 

It  is  written  in  clear  style,  and  presupposes  very  little  knowledge  of 
radio.  The  text  deals  largely  with  the  practical  aspects  of  radio  teleph- 
ony and  its  future.  It  is  copiously  illustrated  with  wiring  diagrams  and 
previously  unpublished  photographs  of  "wireless  telephone"  apparatus. 

IT  IS  THE  ONLY  BOOK  TREATING  THE  SUBJECT  OF  RADIO 
TELEPHONY  IN  ALL  ITS  ASPECTS. 

Among  the  unusual  features  of  the  book  are  a  description  of  how 
radio  telephony  was  carried  on  over  a  distance  of  more  than  5,000  miles ; 
an  illustrated  description  of  an  airplane  radio  telephone  set ;  an  illustrated 
description  of  a  large  ship  radiophone  set ;  numerous  illustrated  sections 
on  smaller  ship  "wireless  telephone"  transmitters;  land  station  radio 
telephone  sets  of  all  sizes. 

Another  noteworthy  feature  is  a  description  of  the  method  of  trans- 
mitting a  radio  telephone  message  to  a  ship  at  sea,  or  across  continent  or 
ocean,  including  the  number  of  persons  involved.  This  material  is  in 
dialogue  form  and  so  worded  as  to  require  no  previous  knowledge  of 
the  subject. 

Among  the  topics  treated  are :  the  construction  and  operation  of  the 
Armstrong  oscillating  audion  circuits ;  the  construction  and  use  of  bulb 
amplifiers ;  the  construction  of  the  great  alternators  of  the  Alexanderson 
and  Goldschmidt  systems  and  how  they  are  controlled,  especially  for 
radio  telephony. 

The  book  is  very  complete,  practically  every  aspect  of  radio  teleph- 
ony being  covered  in  detail.  There^are  over  400  separate  topics  listed  in 
a  carefully  prepared  index. 

8vo.    256  pages.     226  illustrations. 
Full  cloth,  stamped  in  gold.      Price  $2.00  net. 


Specialists  in  \Y7ivo1ooc    Pwaee      IKI/»  25  ELM  ST' 

Wireless  Literature  W  llClcSS   TiCSS,    IIIC.      NEW  YORK,  N.  Y. 


MILITARY  SIGNAL  CORPS  MANUAL 

By  Major  J.  Andrew  White 

Chief  Signal  Officer  of  the  American  Guard,  Member  Institute  of 
Radio  Engineers,  Acting   President    National  Wireless  Association 

This  manual,  the  first  of  its  kind  and  the  only  complete  work  on  the  broad 
subject  of  army  signaling,  is  indispensable  to  those  responding  to  the  call  to  the 
colors.  Primarily  prepared  for  Signal  Corps  men,  it  is  a  necessity  for  the  proper 
understanding  of  apparatus  and  the  tactical  employment  of  troops  and  equipment. 

Officers  of  infantry  and  artillery  will  find  the  volume  of  great  utility,  a  proper 
conception  of  the  enormously  enlarged  Service  of  Information  being  indispensable 
to  all  commissioned  men. 

Its  contents  include  administration  and  government  of  military  units — 
tactics  of  the  division  on  the  march,  at  rest  and  in  engagement — function  and 
operations  of  the  Signal  Corps  and  its  relation  to  the  line  of  the  army — drill 
instruction,  mounted  and  dismounted,  for  telegraph  companies,  radio  and  outpost 
companies,  and  battalions  of  Signal  Corps — signaling  by  telegraph,  heliograph, 
night  lantern  and  flags,  radio  and  service  buzzer — camp  and  field  telephones  and 
their  uses — radio  apparatus  of  the  Signal  Corps — scouting,  patrolling  and  tactical 
employment  of  field  lines. 

Prepared  with  the  full  co-operation  and  approval  of  the  Chief  Signal  Officer,  U.  S.  Army. 

CONTENTS 
Part  I. — Organization 

The  Signal  Corps,  An  Auxiliary  Branch  of  the  Army.  The  Signal  Corps' 
Relation  to  the  Line  of  the  Army.  Aircraft.  Government  and  Administration. 
Military  Courtesy.  Personnel.  Proficiency  Test  for  Companies  of  Signal  Troops. 

Part  II. — Drill  Instruction 

General  Principles.  Definitions.  Commands  and  Signals.  School  of  the 
Soldier.  Physical  Training.  School  of  the  Squad.  Instruction  with  Arms.  The 
Company  Dismounted.  The  Soldier  Mounted.  Elementary  Collective  Instruction 
Mounted.  Field  Signal  Troops.  The  Wire  Company.  The  Radio  Company.  The 
Outpost  Company.  The  Field  Battalion.  Telegraph  Signal  Troops.  The  Tele- 
graph Company.  The  Telegraph  Battalion.  Base  Line  Signal  Troops.  Depot 
Signal  Troops.  Ceremonies.  Reviews.  Inspections.  Funeral  Escort.  The 
Standard.  The  Guidon.  Manual  of  the  Saber. 

Part  III. — Technical  Instruction  and  Apparatus 

Telegraphy  and  Telephony.  The  Voltaic  Cell,  Ohm's  Law,  and  Primary  and 
Secondary  Batteries.  The  Camp  Telephone  and  the  Buzzer.  Induction  Telegraph 
Set.  Radio-telegraphy.  Radio  Apparatus  of  the  Signal  Corps.  Visual  Signaling 
Equipment.  Flag  Kits.  The  Heliograph.  The  Acetylene  Lantern.  Technical 
Equipment  of  Personnel. 

Part  IV. — Transmission 

Transmission  of  Military  Information.  General  Instructions  for  Army  Sig- 
naling. The  American  Morse  Code.  The  International  Morse  or  General  Service 
Code.  Instruction  in  Garrison.  Visual  Stations.  Telegraphy.  Visual  Signaling 
in  General.  Signaling  by  Flag,  Torch  and  Lantern,  or  Beam  or  Searchlight 
(without  Shutter).  Signaling  with  Heliograph,  Flash  Lantern,  or  Searchlight, 
(with  Shutter).  The  Ardois  System.  Signaling  by  Two-Arm  Semaphore.  Sig- 
naling by  Hand  Flags.  General  Instructions  for  Locating  and  Operating  Visual 
Stations.  Letter  Codes.  Telegraph  Code  Books  and  Ciphers.  Conventional  and 
Preconcerted  Signals  with  Rockets,  Bombs,  Small  Arms  and  Guns.  Flag  Signals 
by  Permanent  Hoist.  Emergency  Signals.  Conventional  Telephone  Signals. 

Part  V.— Field  Service 

Signal  Troops  in  the  Field.  Reconnaissance,  Patrolling  and  Scouting.  Mili- 
tary Map  Reading.  Signal  Troops  in  Field  Service.  Field  Lines.  Camps. 
Company  Guard  Mounting.  The  Signal  Corps  and  General  Coast  Defense.  Coast 
Defense  Information  in  War. 

Cloth.      588  pages.      260  illustrations.      Price  $1.50  net. 

Specialists  in  \Y7*      1  D  I  25  ELM  ST- 

Wireless  Literature  W 1161688   r  1688,    IRC.       NEW  YORK,  N.  Y. 


THIS  BOOK  IS  DUE  ON  THE  LAST  BATE 
STAMPED  BELOW 


AN     INITIAL     FINE     OF     25     CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  5O  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


JAN  1  6  1936 


LD  21-20m-6,'32 


YC  33545 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


